Which line is the flattest (or is less steep)? Why?

Question

y = 5x -6
y = 5x + 6
y = x - 3
y = x + 3

Slope is the the coefficent of x. You have a choice here of two slopes: 1, and 5. Either of the two y= x + ? has the same slope.

so is it the last one?

either of the last two have a slope of one. Both have the same "steepness".

Margie, I've seen this question posted a couple time and answered correctly too. I answered it below, but my answer was deleted for some reason unknown to me.
There is no "flattest (or is less steep)" line among the answers.
The first two lines are parallel with a slope of 5.
The last two are also parallel to each other, with a slope of 1, therefore the last "two" are less steep than the first two, but there is no "least steep line" since they're parallel.
Graph them to verify this. The question is stated incorrectly if it was intended to have a single correct answer.