Explanation

(d) Maximum amplitude, $A_{\max} = A_{c} + A_{m}$ …(i)
Minimum amplitude, $A_{\min} = A_{c} - A_{m}$ …(ii)
Solving (i) and (ii), we get
$A_{c} = \frac{A_{\max} + A_{\min}}{2},\ A_{m} = \frac{A_{\max} - A_{\min}}{2}$
Modulation index, $\mu = \frac{A_{m}}{A_{c}} = \frac{A_{\max} - A_{\min}}{A_{\max} + A_{\min}}$
$= \frac{25\ V - 5\ V}{25\ V + 5\ V} = \frac{20}{30} = \frac{2}{3}$