A carrier wave of peak voltage $12 \mathrm{~V}$ is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75\%?
(a) $7 \underline{\mathrm{V}}$
(b) $6 \mathrm{~V}$
(c) $9 \mathrm{~V}$
(d) $8 \mathrm{~V}$