Question

it is difficult to make___what he writes. (preposition)

Answer 1

Fill in the blanks with a proper preposition:

      It is difficult to make ___ what he writes.

Answer:
It is difficult to make out what he writes.

"Make out " means to understand, analyse, recognise , identify, read or comprehend.

Answer 2

It is difficult to make out what he writes.
It's the correct answer.