In the passage about transformational technical skills, which word best completes the sentence: "In the first place, he must know ______ skill and what level of skill should be applied on a particular student"?

Introduction / Context:
This question is taken from a passage about technical skill and transformational teaching. The line being tested describes a teacher who needs to choose the correct technical skill and level of that skill for each student. The blank tests knowledge of relative interrogative words such as which, when, while, and who, and how they fit into a complex noun clause in formal English.

Given Data / Assumptions:

• The key sentence is: He must know ______ skill and what level of skill should be applied on a particular student.
• The teacher is choosing between different possible skills.
• The four options are when, which, while, and who.
• The blank must complete a noun phrase that functions as the object of must know.

Concept / Approach:
In English, when we select from a known set or range of items, we use the word which. The clause which skill and what level of skill should be applied describes exactly what the teacher must know. The word when is used for time, while for contrast between two clauses, and who for people. None of those words correctly introduces a choice among different skills. The structure must know which skill and what level of skill is grammatically and logically correct, because which directly refers to selection among alternatives.

Step-by-Step Solution:
Step 1: Identify the verb and its object: must know (something). That something is a complex noun phrase beginning with the blank.Step 2: Notice that skill is a countable noun here, implying a choice from several possible skills.Step 3: Recognise that which is the standard determiner used when choosing between known or limited options, as in which book or which method.Step 4: Test when in the blank: when skill and what level sounds wrong because when introduces time, not selection.Step 5: Test while and who; while skill is not grammatical, and who skill is impossible because who refers to persons, not to skills. Therefore, which is the only word that yields a natural, meaningful clause: which skill and what level of skill should be applied.

Verification / Alternative check:
Read the completed sentence aloud: There are two things a teacher should understand regarding transformational technical skills. In the first place, he must know which skill and what level of skill should be applied on a particular student. This sounds like clear, standard academic English. If we insert any of the other options, the sentence becomes either ungrammatical or illogical. For example, must know when skill is impossible; must know who skill wrongly personifies skill; must know while skill cannot function as a noun phrase at all. Only which fits both grammatically and semantically.

Why Other Options Are Wrong:
When introduces clauses of time, such as when the lesson begins, and cannot be used as a determiner with skill in this way. While is used to contrast two actions or situations and does not modify nouns directly. Who is a pronoun used for people, not for abstract things like skills, so it cannot stand before skill. None of these words can introduce a meaningful object for must know in this sentence.

Common Pitfalls:
Some learners confuse which and what in complex noun phrases. Here, however, the exam does not even offer what, forcing you to focus on the clear choice. Another pitfall is reading too quickly and thinking when might fit because teaching and timing are connected, but the grammar of the phrase immediately rules that out. Training yourself to check both meaning and sentence structure will help you avoid such traps.

Final Answer:
The sentence should read he must know which skill and what level of skill should be applied, so the correct option is which (option B).