B. Non zero
I had been asked this question by my college professor while I was bunking the class.
This interesting question is from Parameters of Mobile Multipath Channels in portion Mobile Radio Propagation: Small – Scale Fading and Multipath of Wireless/Mobile
The explanation is: Doppler spread is a range of frequencies over which received Doppler spectrum is essentially non-zero. It is a measure of spectral broadening caused by time rate of change of mobile radio channel.