so how did we eliminate the other options? And is there an easier and quicker way to do this rather than trial and error? I also had a hard time understanding the wording of the question

Since this is a select one multiple choice ans there cannot be more than 1 ans choice. If in the process of solving the problem we get our ans, no need to work with other options.
There can be other methods for solving the ans for eg. you can formulate a eqn \(x(x-1) = 2a\); where a is the ans options
make the left side equivalent to the right side for correct ans such as \(x(x-1) = 4*3\) for option B try it with different options only B is correct
Realize that this is not a time consuming question. If you understand the logic the question should not take more than 1'30 min to answer
Wording of questions can be confusing, ets regularly uses this technique to make test-takers chose wrong answers. Get familiar with gre language. Give attention to sign words such as only, not, must be, could be, if, approximately and so on