The correct word to fill in the blank is "when". Let's break down the options and explain why "when" is the correct choice:
Option A: "when"
- "When" is used to refer to a specific time or period.
- In this context, it indicates the time at which the speaker and the person they're addressing went to the Chinese restaurant.
- This option is correct because it fits the sentence structure and conveys the intended meaning.
Option B: "whom"
- "Whom" is used as the object of a verb or preposition, usually referring to a person.
- In this context, "whom" is not appropriate because it does not refer to a time or period.
Option C: "which"
- "Which" is used to refer to a thing or things.
- In this context, the speaker is referring to a specific time, not a thing.
Option D: "who"
- "Who" is used to refer to a person or people.
- In this context, the speaker is referring to a time, not a person.
To summarize, the correct word to fill in the blank is "when" because it refers to a specific time or period.

Explanation:
The correct answer for the given sentence is option 'a' - when.

Explanation in detail:
To determine the correct answer, let's analyze the sentence structure and the meaning it conveys.

The sentence is: "I really enjoyed the time _______ we went to that Chinese restaurant."

In this sentence, we need to fill in the blank with the appropriate word that indicates the time when the speaker enjoyed going to the Chinese restaurant.

Let's examine the options:

a) when
b) whom
c) which
d) who

a) when:
The word "when" is used to refer to a specific time or occasion. In this sentence, "when" is the most suitable option because it indicates the time or occasion of going to the Chinese restaurant.

b) whom:
The word "whom" is used as the object of a verb or preposition to refer to a person. It is not appropriate in this sentence because we are not referring to a person, but rather the time or occasion.

c) which:
The word "which" is used to introduce a relative clause and refers to a thing or a situation. However, in this sentence, we are not referring to a thing or a situation, but rather the time or occasion.

d) who:
The word "who" is used to refer to a person or persons. Similar to option b, it is not appropriate in this sentence because we are not referring to a person, but rather the time or occasion.

Hence, the correct answer is option 'a' - when, as it correctly indicates the time or occasion of going to the Chinese restaurant.