In the following passage based question, select the correct form of the verb to complete the blank: The phrase consists usually of a few words which denote a single idea that ________ a separate part of a sentence.

Introduction / Context:
This question is drawn from a passage that explains the grammatical concept of a phrase. It asks the learner to select the correct form of the verb that best completes the sentence: The phrase consists usually of a few words which denote a single idea that ________ a separate part of a sentence. The focus is on subject verb agreement and on choosing the grammatically appropriate form for a relative clause in formal English.

Given Data / Assumptions:

• The sentence subject is a single idea that acts as a unit.
• The blank follows the clause a single idea that, which introduces a relative clause about the idea.
• The options are form, forms, formation, and forming.
• We assume standard present tense description in a general statement about grammar.

Concept / Approach:
In English, verbs must agree in number with their subjects. Here, the subject of the embedded clause is idea, which is singular. The relative pronoun that refers to this singular noun and therefore requires a singular verb. Among the options provided, forms is the singular present tense verb form that agrees with idea. The other options either reflect plural verb form or non finite or noun forms and cannot correctly complete a simple present tense statement defining a phrase.

Step-by-Step Solution:
Step 1: Identify the subject of the clause after that. It is the noun idea. Step 2: Note that idea is a singular noun, so any present tense verb that follows must also be singular for correct subject verb agreement. Step 3: Examine option A, form, which is the base or plural form and therefore does not agree with the singular subject idea in a simple present context. Step 4: Examine option B, forms, which is the third person singular present form and therefore agrees with the singular subject. Step 5: Examine option C, formation, which is a noun and cannot serve as a finite verb in this position. Step 6: Examine option D, forming, which is a present participle or gerund and would require an auxiliary verb to fit grammatically. Step 7: Conclude that forms is the only verb form that correctly completes the sentence.

Verification / Alternative check:
Read the full sentence with the chosen option: The phrase consists usually of a few words which denote a single idea that forms a separate part of a sentence. This reads smoothly and clearly, reflecting a general definition. Substituting any of the other options makes the sentence ungrammatical or awkward. For example, that form a separate part creates a number mismatch, while that formation or that forming a separate part distorts the structure of the clause. Hence, forms is verified as the correct answer.

Why Other Options Are Wrong:
Form is incorrect in this position because a singular subject like idea requires the singular verb forms, not the base form used with plural subjects or modal verbs. Formation is wrong because it is a noun and does not function as the predicate verb. Forming is wrong because, without an auxiliary such as is, it cannot serve as the main verb for the clause. Each of these wrongly changes either the grammatical function or breaks subject verb agreement within the sentence.

Common Pitfalls:
Learners sometimes focus only on the words closest to the blank and may mistakenly match the verb to words like words, which is plural, rather than to the correct subject idea. Another frequent mistake is choosing participle or noun forms simply because they sound similar. The best way to avoid such errors is to identify the true grammatical subject of the clause and always check number agreement and verb type before selecting an answer.

Final Answer:
The correct verb is forms, giving the sentence: a single idea that forms a separate part of a sentence.