The question is asking for unit of length. Of the units given in the five choices, only centimeters is a unit of length, and ten centimeters—the metric equivalent of about four inches—is a reasonable length for a candy bar.

Of the other units:

The kilogram is a unit of mass which corresponds to a little over two ounces.

The hectare is a unit of area, usually land, and is equivalent to about two and a half acres.

The milliliter is a very small unit of volume.

The kilowatt is a unit of power.