My parents taught me a lot of lessons, most of … are difficult to forget.

Analyze the question The sentence is: "My parents taught me a lot of lessons, most of __ are difficult to forget." We need the relative pronoun that correctly follows the preposition "of" and refers back to "lessons."

Explain the concept A relative pronoun links a clause to a noun that came before it (the antecedent). Choosing the right one depends on two things:

• Which refers to things or animals.
• Who/whom refers to people. Whom is the object form, used after prepositions when the antecedent is a person.
• What does not have an antecedent. It means "the thing that," so it cannot follow a noun like "lessons."

Phrases like "most of which," "some of whom," and "many of which" are common patterns: quantity word + of + relative pronoun.

Step-by-step solution

1. Find the antecedent: "lessons." This is a thing, not a person, so who and whom (options C and D) are out.
2. Option B, what, cannot be used here because the clause already has a noun it is describing ("lessons"). "What" would be incorrect.
3. Test option A: "...a lot of lessons, most of which are difficult to forget." This is grammatically correct. "Which" refers to "lessons," and the plural verb "are" agrees with "most of which" (meaning most of the lessons).

Answer: A. which

Exam tip: In the pattern "most/some/many/all of ____," ask yourself whether the noun before it is a person or a thing. People → whom; things → which. Also, "what" almost never follows a noun directly in relative clauses.