Read the sentence: joel and lucy expect blank space halloween party to be really scary! which pronoun best completes the sentence?

It causes a little confusion to have that "Blank Space" right after the the main verb "expect" because we cannot be sure that it represents that a blank or a space is that follows, or that Blank Space is used as a noun.

To answer this question I will assume that blank space represents that a line goes there to be filled.

By doing so, the correct sentence would be:

Joel and Lucy expect _their_ Halloween party to be really scary!

So the final answer is:

Their

The possessive third person plural pronoun is the best option.

can you elaborate on what the question asks?