The vertical axis is the same as the y axis and the horizontal axis is the same as the x axis. You're told that the y/vertical axis is inches and the x/horizontal axis is feet. Remember that there are 12 inches in a foot. That means y (number of inches) = 12x (12 times number of feet, x). So your equation is y = 12x.
You can plug some numbers into y = 12x, just to make sure this is the right equation. Let's try x = 1 foot. When x = 1, y = 12(1) = 12 inches. Yep! Let's try x = 2 feet. When x = 2, y = 12(2) = 24 inches. Yep!
That means y = 12x is the correct equation for your graph.
Remember that the general equation for slope-intercept form is y = mx + b. And m = the slope of the equation. (b = the y intercept, and x and y are your variables, aka the coordinate points on the graph, but that's not important in this question). You want the value of m, because that is the slope of the graph. Notice that m is multiplied by x. In our equation, y = 12x, 12 is being multiplied by x. That means 12 is m, our slope!
The slope of the graph is F) 12.