Fill in the blank with a suitable preposition-
Who’s the blonde girl ............... the first row?

We always place the topic and verb before the preposition, followed by the noun.
Example- The boy is sitting beside the principal. Here, the topic ‘boy’ and therefore the verb ‘sitting’ are placed before the preposition ‘beside’, followed by the noun/object ‘principal’.
In the above question, ‘X’ is enquiring about the blonde girl within the first row. So, X has indicated the situation or the place.
Let’s examine the given options to search out suitable preposition:
a)In- it's wont to tell the place or location of a noun.
Example- She sat within the train.

b)On- it's wont to indicate the surface or aspect of something.
Example- The pen is on the table.
c)At- it's accustomed tell the place of a noun.
Example- i'll meet you at the library.
d)Over- it's wont to specify movement at the next position.
Example- A bird flew over the river.

X has mentioned the placement or the place of the blonde girl. Hence, the right answer is Option ‘a’.

Note: ‘In’ is additionally wont to specify months, seasons, years, and decades.