Question

A: Where did you have......lunch? fill with a,an,the​

Answer 1

'No article' is used to before abstract / uncountable nouns such as 'lunch'. 'A' is used before singular nouns used in a general sense. 'Restaurant' .

Answer 2

no extra letter is needed

Step-by-step explanation:

The sentence is fine by itself because the addition of a, an, or the would sound redundant and wouldn't make sense.