The correct answer is C. think.
The word "ponder" means to think deeply about something. It is often used in the context of something that is difficult to understand or that requires careful consideration. In the sentence "How beautiful! This is really something to ponder over," the speaker is expressing their admiration for something and their desire to think about it more deeply.
The other options are not as appropriate in this context. Singing and dancing are both activities that are typically done for enjoyment, rather than for deep thought. Wondering is similar to pondering, but it can also be used in a more casual sense to mean "to think about something briefly." In this case, the speaker's use of the word "really" suggests that they are thinking about the beauty of the thing they are describing in a very serious way.
