Answer:
C
Explanation:
Notice that the 5 is located under the column marked "seconds", so it must have the units of seconds; in other words, we have 5 seconds of something. That already eliminates A and B because they say that 5 is words.
However, let's verify. The 5 seconds corresponds to the 1, which is in the same row as the 5 but under the column "words typed". In other words, in 5 seconds, Casey typed 1 word.
Thus, C is the answer.
Hope this helps!