Answer: Option A
The given sentence is in the past tense and describes an action that was completed in the past. The sentence is correctly structured, but the use of the word "were assembled" is not ideal in this context. We can replace it with a better phrase to make the sentence grammatically correct.
Option A, "had assembled to celebrate," is the correct answer. Here's why:

• The past perfect tense "had assembled" is used to describe a completed action that took place before another action in the past. In this case, the action of "assembling" happened before the action of "celebrating."
• The word "to" is the correct preposition to use with "celebrate," indicating the purpose of the assembly.
• The phrase "to celebrate" is in the infinitive form, which is appropriate to use after the verb "assembled."

Let's look at the other options:

• Option B, "were assembling to celebrate," is incorrect because it implies that the action was ongoing and incomplete. This doesn't accurately describe the situation because the members had already assembled.
• Option C, "had been assembled for celebrating," is incorrect because the phrase "for celebrating" is not grammatically correct. We would need to use "to celebrate" instead.
• Option D, "assembled to celebration," is incorrect because "celebration" should be "celebrate" to match the infinitive form.
• Option E, "No correction required," is incorrect because we do need to make a correction to the sentence for better grammar.

In conclusion, the correct answer is Option A, "had assembled to celebrate," because it accurately describes a completed action in the past and uses the correct preposition and infinitive form.