Which of the following lines is parallel to x = 8?

Interestingly enough, the line x=8 is parallel to the y axis.  The line x=8 means that x=8 no matter what y equals.  
So any line that is parallel to the y axis could be the answer.

x=4
x=12
x=-17
x=-2.5   are all examples of lines that are parallel to x=8.  Perhaps that will answer your question.

the formula of a slope:

from a table we have

substitute:

the slope of f(x) is -1.

the graph of funcfion g(x) is a horizontal line. the slope of a horizontal line is equal 0. therefore the slope of g(x) is 0.

-1 < 0

answer: the function g(x) has the greatest slope.