Assertion : In case of projectile motion, the magnitude of rate of...

Solution: The assertion is false, and the reason is true. - Assertion: The rate of change of velocity is acceleration. In projectile motion, ignoring air resistance, the acceleration is due to gravity ($g$), which is constant. Therefore, the magnitude of the rate of change of velocity is constant, not variable. So, the assertion is false. - Reason: The magnitude of velocity (speed) in projectile motion is given by (|v|=\sqrt{v_x^2+v_y^2}). The horizontal velocity ($v_x$) is constant, but the vertical velocity ($v_y$) first decreases to zero at the peak and then increases. This causes the overall speed to decrease on the way up and increase on the way down. So, the reason is true. The correct option is (4), "If Assertion is false but Reason is true."