It's are. When two things are connected by an "and" the resulting compound subject is plural.
<p>First of all, this doesn't look like an SAT problem. Don't use the book or whatever study source you got the problem from. Just stick to the blue book.</p>
<p>"Planning a surprise party and keeping it secret" acts as a singular element because the two actions are grouped together. They are essential to one another. If they were treated as separate elements (in which case the subject WOULD be plural) then there would be an implication that planning a surprise party is a difficult endeavor AND keeping the party secret is a difficult endeavor. The sentence doesn't mean to say that planning a party by itself is a difficult endeavor, nor does it mean to say that keeping the party secret by itself is a difficult endeavor. It means to say that doing both collectively is a difficult endeavor. You can't have one without the other, so you treat the whole thing as one singular thing. If the subject were plural the object would have to be plural as well: "Planning a surprise party and keeping it secret are always difficult endeavors."</p>
<p>There's nothing wrong with the rest of the sentence.</p>