If acceleration is constant (and apparently positive), then velocity increases linearly, so the graph of velocity with respect to time is a line with positive slope. The region between this line and the horizontal axis would then be a triangle or a trapezoid, depending on the object's velocity at the start (which we don't necessarily know). To get the object's displacement, one would have to integrate the velocity, which in terms of the graph corresponds to the area of this triangle/trapezoid.
But the given options are quite vague... We can rule out the first two, since they seem to use "line" to refer to a constant function with 0 slope, which we know is not the case with the velocity graph in this situation.
The other two options are ambiguous as well, but it seems like the third is the right one.
