Although the square root of a negative number has no real value, it is not necessarily true that equations involving imaginary numbers like these are practically inapplicable.

The idea conveyed by the whole sentence is that while the square root of a negative number has no real value, equations involving such numbers may have practical applications. Due to the usage of "not necessarily," the answer we are looking for should be phrased as "such equations have no practical applications" and the double negation will give the sentence a positive essence.

The only answer that conveys the required sense is B.