Step 1: Define a circular argument.
A circular argument (begging the question) is one where the reason given simply restates the conclusion, so it offers no independent support.
Step 2: Examine option 1.
"If it rains the grass gets wet; the grass is wet so it rained" is affirming the consequent, a different flaw, since it supplies an outside premise about rain and wetness.
Step 3: Examine option 2.
"Free speech is important because people should be able to say what they want." The reason just rephrases the meaning of free speech, so the conclusion is used to justify itself. This is circular.
Step 4: Examine option 3.
"Most people buy this brand so it must be the best" appeals to popularity, a bandwagon flaw, not circularity.
Step 5: Examine option 4.
"He is a good doctor because he graduated from a top university" gives an external (if weak) reason, so it is not circular.
Step 6: Conclude.
Only option 2 restates its own conclusion as its reason, so it is the circular argument.
\[ \boxed{\text{Free speech is important because people should be able to say what they want}} \]