The modulation index of a single tone FM signal is 2π rad. If the frequency and the amplitude of the message signal are increased by a factor of 2, then the modulation index of the modified FM signal will be equal to
β=kfAmfm
β1=kfAm1fm1
β2=kfAm2fm2=kf(2Am1)(2fm1)=kfAm1fm1=β1=2π rad