The correct answer is: A. Clubbing adjacent radio channels.
CDMA2000 is a 2G and 3G cellular network technology that uses code-division multiple access (CDMA) to allow multiple users to share the same radio frequency (RF) channel. CDMA2000 can increase bandwidth by clubbing adjacent radio channels together. This is done by assigning two or more radio channels to a single user, which allows them to transmit data at a higher rate.
Option B is incorrect because changing the hardware of base stations would not increase bandwidth. The hardware of base stations is already designed to support CDMA2000.
Option C is incorrect because changing the spectrum would not increase bandwidth. The spectrum is the range of frequencies that are available for use by cellular networks. Changing the spectrum would not allow CDMA2000 to transmit data at a higher rate.
Option D is incorrect because changing the RF equipment would not increase bandwidth. The RF equipment is already designed to support CDMA2000.