In the following passage, some of the words have been left out. Read the passage carefully and select the correct answer for the given blank from the four alternatives. "Jim Crow" shuns the mountains for reasons satisfactory to himself; not so the magpie, the raven, and a certain mischief maker, Clark's nutcracker. All of this keeps the bird lover from the East in an ecstasy of surprises until he has become fully accustomed to his changed environment. One cannot help falling into a speculative mood in view of the sharp contrasts between the birds of the East and __________ of the West. Choose the correct demonstrative pronoun to fill the blank in the birds of the East and __________ of the West.

Introduction / Context:
This question comes from the same descriptive passage about different kinds of birds in the Eastern and Western regions. Here, instead of a preposition, the test focuses on demonstrative pronouns. The sentence structure compares the birds of the East with another group of birds belonging to the West. The correct demonstrative pronoun must refer back to birds and must agree in number and distance in a natural way.

Given Data / Assumptions:
The relevant part of the passage reads the birds of the East and _________ of the West. We know that birds is a plural countable noun. It appears first as the birds of the East, and then another blank appears before of the West. The options available are whose, this, those, and whom. We assume that the exam expects standard modern written English and that the blank is to be completed by a single word that refers to birds of the West.

Concept / Approach:
In English, demonstrative pronouns like this, that, these, and those are used to point to specific people or things. When referring back to a plural noun that has already been mentioned and that is slightly distant in idea, writers generally use those. In the pattern X of the East and those of the West, the word those clearly stands for those birds. The other options function differently: whose is a possessive relative pronoun, this is singular and usually refers to something near, and whom is an object relative pronoun used for people, not for birds in this construction.

Step-by-Step Solution:
Step 1: Identify the noun being referred to in both parts of the phrase, which is birds. Step 2: Notice that the full logical meaning is the birds of the East and the birds of the West. Step 3: Recognise that to avoid repetition, English often uses a demonstrative pronoun instead of repeating the noun. Step 4: Among the options, choose the demonstrative pronoun that can stand for a plural noun at some conceptual distance: those. Step 5: Read the completed phrase the birds of the East and those of the West to confirm that it is natural and grammatically correct.

Verification / Alternative check:
A quick check is to expand the sentence mentally: the birds of the East and those of the West really means the birds of the East and the birds of the West. If we replace birds with people, we might say the people of this city and those of that city, which sounds standard and natural. None of the other choices fit such patterns. For example, the people of this city and whom of that city is clearly wrong, and the people of this city and this of that city is also ungrammatical. Therefore, the pattern strongly supports using those in this position.

Why Other Options Are Wrong:
Whose is a possessive relative pronoun and would normally begin a clause such as whose feathers are bright, so it cannot stand alone before of the West here.
This is singular and would refer to a single bird or a single idea, which clashes with the plural sense of birds and the contrast between two large groups.
Whom is used for people in the object position of a clause, for example the person whom I met, and it cannot correctly refer to birds in this abbreviated noun phrase structure.

Common Pitfalls:
Learners sometimes confuse demonstrative adjectives and demonstrative pronouns. They may also forget that this and that are singular while these and those are plural. Another frequent error is to pick a word simply because it seems familiar, without checking whether the grammar pattern is complete. In competitive exams, paying attention to number agreement and typical constructions such as those of the West is essential. Remember that when English avoids repetition of a plural noun mentioned earlier, those is often used to stand for that noun in a concise way.

Final Answer:
The correct demonstrative pronoun to complete the phrase the birds of the East and _________ of the West is those.