Some part of the sentence may have errors. Find out which part of the sentence has an error and select the appropriate option. If a sentence is free from error, select 'No Error'.
"To the whale, its(A)/tail is the sole(B)/mean of propulsion.(C)/No error(D)"

The error lies in part C of the sentence. The singular noun "mean" is incorrect in this context. When referring to a method, instrument, or mechanism by which something is done, the correct noun form is "means" (which is the same in both singular and plural forms).
Therefore, "sole mean" should be corrected to "sole means".