The prefix for the multiple ${10}^{18}$ is 

The prefix for the multiple 10¹⁸  is Exa.

A unit prefix is a specifier or mnemonic that is added to units of measurement to indicate multiples or fractions of the units. Units of various sizes are routinely created using these prefixes. The prefixes of the metric system, such as kilo and milli, denote division by powers of ten.

The prefix Exa, Giga, Nano, and Kilo represents ${10}^{18},{10}^9,{10}^{-9},{10}^3$.

So the correct answer is (c).