Premise 1: parties happen at the same time, so hosts use visual ads to attract ppl.
Premise 2: a company that produces visual ads is located in LA.

Conclusion: LA is very crowded with parties.

There is a pretty big gap in the reasoning. Parties might happen in any other place, not in LA necessarily. Conclusion will only be sound if premise 1 and premise 2 are connected.

(A) There is also a business in Los Angeles that produces ordinary party invitations.
- Incorrect. Okay, cool. We are concerned with flashy visual ads.

(B) Party guests can attend no more than one party per evening.
- Irrelevant. Alright. Why does it matter? How does it relate to the argument?

(C) At most parties, there are uninvited guests who add to' the number of people in attendance.
- Irrelevant. Again, there is no connection to the argument.

(D) Many of the visually striking party invitations produced in Los Angeles are bought for parties in Los Angeles.
- Correct. Definitely addresses the premises of the argument.

(E) Sending party invitations is a duty that hosts rarely delegate to others.
- Irrelevant. Who cares about sending the invites?...