Future tense or future perfect tense, which one should be used in this sentence?

by 2050, time travel .... possible.

a) will be

b) will have been

which answer is correct? is it wrong to use "will be" in that case?

"Will have" is future perfect tense and refers to something in the future that has been completed, but you want to refer to something (time travel) that will be ongoing in the future, so you need future progressive.

Therefore, 'a' is correct, at least grammatically:

By 2050 time travel will be possible.

You would use "will have been" if you were referring to something completed that allows for the future possibility, for example:

By 2050 the time machine will have been invented.

or simply:

By 2050 time travel will have been made possible.