This is a question you could solve by process of elimination. The lowest dip is between 1000 Hz and 10,000 Hz, meaning the ear is most sensitive to frequency values between those numbers. The only answer choice to reflect that is C.
Sound level in dB is a log scale. Remember our equation as dB = 1o*log(I/Io), however that's related to intensity. Though while the graph appears to be a semi-log graph, without looking at the passage to see if frequency is being treated in a similar way here, this equation, and this observation that the graph is semi-log, is not relevant. Best bet, considering timing as well, is to use process of elimination here.
