Because they hid the people and made sure they were the only ones who got to keep tabs
The correct answer is that it is a dependent clause.
A dependent clause, as the name itself says, depends on a larger clause in order to make sense. As you can see in the example above, that clause is incomplete - it needs to be part of a larger, independent clause in order to get a proper meaning. The other options are all complete sentences, which is something the example is definitely not.
A guilty conscience is not easily mended.
I believe the answer is Admission to the park is $48 per person, but a beach pass is only $3. %^