Question

If our party - to be fun, I think our plan needs a lot attention.
(a) was planning
(b) will
(c) 's going
(d) 's been​

Answer 1

If our party is going to be , I think our plan needs a lot attention .

Answer 2

If our party is going to be a fun, I think our plan needs a lot.

Explanation:

I chose this option besides other one because it reflects toward future i.e. if our party is going to be a fun; I think our plan needs a lot. The meaning of this sentence is that we need to have more plans if we want our party to be a fun.  

That’s the reason, I choose future tense. Although “will” also present in the given options but we never put "will" with "to" because It doesn’t make sense.

Hope it helped..............