Question

At the beginning of the mission, since our priority was to identify the origin of the alien’s signal.

Answer 1

The most accurate way to describe the excerpt is to choose the option - it is a dependent clause. This means that something needs to be added - you need to finish the sentence so that it becomes an independent clause, and not a subordinate one. 

Answer 2

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.