Ok, so first we must understand what the slope of a line actually is. The slope, or gradient, is the value by which y increases (or decreases, if this is negative) for each increase of one unit in the x-value. So basically what you should think about for this question is, as x increases, does y decrease, increase or stay the same?
From the graph, we can see that as the value of x gets larger, the value of y also increases, therefor the slope is positive.
For the other answers:
If the slope were negative, we would see that the y-values would get smaller as x would get bigger.
If the slope was zero, you would see a horizontal line; this means that there is no change in the y-value as x-increases.
If the slope was undefined, you would see a vertical line; this is an extreme case where you cannot say how much y increases by as the line continues till infinity, therefor we say that it is undefined.