The BEST classification for -15 is
Whole number
This is because fractions and decimals can also be classified as Rational and Natural numbers, but not as whole numbers.
Note that it is also an integer, that is, -15 can be classified as both a whole number and as an integer, but for the sake of this particular question, only one answer can be tendered. As per definition, a whole number needs little or no further explanation, hence the choice to classify it best as a whole number