A. The kitten, lapping the cold milk, must be the hungriest.
The kitten must be the hungriest is a sentence by itself. Adding "lapping the cold milk" gives meaning to "hungriest", but is not needed. It is "extra information"
C. The boy hurt his left arm playing volleyball yesterday.
The two sentences with commas did not use them correctly, and so C is your best answer
hope this helps