An idea about why something happened or how it works is known as a(an)
a. agreement
b. fact
c. theory
d. phenomenon

The answer is indeed C. theory.

Explanation:

Scientifically speaking, a theory would be a formal set of ideas or a supposition whose intention is to explain something based on principles not directly connected to that something. In our everyday life, however, the word theory is frequently employed in a lighter way, but basically with the same meaning. If we have an idea, based on our own suppositions or any sort of information we might have, that explains a happening or a behavior, then we say we have a theory.

Imagine the following situation: my boyfriend is late for our date for the first time. Earlier, he had told me he was in college trying to finish a paper with some classmates. He also told me they were having problems wrapping it up because no one had actually finished reading the material they needed. The paper is due tomorrow. From the previous information, I have a theory that he stayed at the library working on his paper longer than he had intended, since he had no option but to finish it, and that is why he is late.

a. the man as a ruler

first, you have to go to settings, click on your profile, go to profile settings, and you'll find the setting there.