Explanation:

To answer this question, let's break down the given sentence and analyze the options provided.

The sentence states: "This room is not big ____ for both of us to study our lessons."

The word "big" suggests that the room does not have enough space for both individuals to study comfortably. Therefore, we need to find an option that indicates the room's insufficiency in size.

Option 'A': "at all"
- This phrase is used to emphasize a negative statement, but it does not convey the idea of the room's size.
- It does not fit in the context of the sentence, so it is not the correct answer.

Option 'B': "enough"
- This word directly addresses the issue presented in the sentence.
- It indicates that the room lacks sufficient space for both individuals to study comfortably.
- Therefore, option 'B' is the correct answer.

Option 'C': "just"
- This word suggests a small amount or limited space, but it does not clearly convey the idea of insufficiency.
- It does not fit in the context of the sentence, so it is not the correct answer.

Option 'D': "nearly"
- This word means almost or close to a certain state or condition.
- It does not convey the idea of the room's size or insufficiency.
- It does not fit in the context of the sentence, so it is not the correct answer.

Conclusion:
In conclusion, the correct answer is option 'B' - "enough." This option accurately describes the room's lack of sufficient space for both individuals to study their lessons comfortably.