(d)

If velocity is changing they may change in magnitude or direction or both.

$(i)$ So, if velocity is changing in direction only the magnitude is constant so speed is constant.

$(ii)$ If only direction of velocity is changing and magnitude is constant then acceleration will also be constant in magnitude (in case of uniform circular motion).

$(iii)$ Average acceleration may be constant. $a_{ av }=\frac{v_2-v_1}{t_2-t_1}$