which is the subordinate clause in the sentence?
some unprotected animals get swept out of the tide pool when waves rush in with much-needed moisture.
a. out of the tide pool
b. with much-needed moisture
c. some unprotected animals get swept out of the tide pool
d. when waves rush in with much-needed moisture

A subordinate clause is a sentence that is started by a subordinating conjunction, for example "when the phone rang." 
It is also called a dependent clause. It cannot stand alone as a sentence on its own, due to the subordinating conjunction "when" in this case.

Some subordinating conjunctions are when, after, since, where.
So in your sentence, you see "when," a subordinating conjunction.
So your subordinate clause is when waves rush in with much-needed moisture

First find the subordinate conjunction. In this case, it is "when". The subordinate conjunction leads to the subordinate clause, which is this case, then 
D. when waves rush in with much needed moisture. 
Hope this helps.