The phrase "The hippos look _____" requires an adjective or adverb to complete it properly.

Among the options:

A. MERRILY - This is an adverb which typically describes how an action is performed, and it's not commonly used to describe how something "looks."

B. HAPPILY - Similar to "merrily," this is also an adverb, and while it describes a state, it doesn't fit well with the verb "look."

C. STRONG - This is an adjective and can describe the physical appearance of the hippos, making it a suitable option.

D. LOUD - This is also an adjective but describes sound, which does not apply to how hippos look.

The best fit here would be:

**C. STRONG**