Question

He ______this story tomorrow. (write)

Answer 1

HELLO DEAR FRIEND

He
$\underline{will \: write}$ this story tomorrow. (write)

Answer 2

He ______this story tomorrow.

If you ask me to fill up this blank with a form of 'write', I would do it like this:

He wrote this story tomorrow.

This sentence is written in simple past tense. It is a basic form of past tense in which subject is preceded by second form of the verb.

When 'write' is the verb, it's second form of verb would be 'wrote'.