imc

(a) Triangles $ABP$ and $MNP$ are similar, and since $PM=\frac{1}{2}AP$, $MN=\frac{1}{2}AB$. (b) We see the perimeter changes. For example, imagine if P was extremely far to the left. (c) The area clearly doesn't change, as both the base $AB$ and its corresponding height remain the same. (d) The bases $AB$ and $MN$ do not change, and neither does the height, so the area of the trapezoid remains the same. Only $1$ quantity changes, so the correct answer is $\boxed{1}$.