Answer:
The correct answer is "As soon as we had finished dinner (it was about 6:30 p.m.), I started my homework."
Step-by-step explanation:
In this case, parenthesis are used to add information to the sentence that is not essential to its meaning. If we remove the information in the parenthesis, the sentence will still have meaning on its own.
The rest of the options are incorrect since if we remove the information in parenthesis, the sentence does not have meaning. For example, in option 1, if we remove (6:30 pm) the sentence is grammatically incorrect since it would read as follows: "As soon as we had finished dinner it was about, I started my homework."