The successor of – 18 is

A successor is a term that appears immediately after a number, term, or value. 

If n is a number (and n might be any whole number), then n + 1 is the number that comes after it.

For example, If, n = - 10 then successor of - 10 is - 10 + 1 = - 9

In the successor we have to move from left to right on the number line.

Also, - 17 is on the right side of - 18.

As a result, - 17is the successor of – 18.

Hence, the correct option is (c) i.e. - 17.