Since there are no clear choices listed I will try to answer to the best of my ability.
A "Variable" is an inconsistent set of numbers. This set is subjected to change over time. The most common set or best demonstration would be an "input-output' Variable.
x = 0
y = 1 + 2x Substitute x = 0
y = 1 + 2(0) Any number times 0 is 0
y = 1 + 0 Simplify
y = 1
I hope this could help.