Question regarding CDMA Rake receiver

The book says..

A CDMA Rake Receiver is able to exploit multipath when the channel is ( kindly select appropriate one)

(a) Flat fading (b) Slow Fading (c) Fast Fading (d) Frequency Selective Fading


kindly suggest me which are appropriate

I think (a) should be one of the answers.

---------- Post added at 17:21 ---------- Previous post was at 16:22 ----------

Just to correct my first answer. It can be (d) instead of (a), because RAKE receiver (R. Price & P.E. Green, ’A Communication Technique for Multipath Channels’, Proc. IRE, vol. 46, 1958, pp. 555-570) exploits multipath diversity to combat fading, thanks to a fundamental time-frequency decomposition of the channel into independent flat fading channels. Normally the question is asked by choosing all that apply (selections a,b,c,d). Here, the worst situation is when we have a frequency selective/fast fading channel. The scenario which suits CDMA Rake receiver. Therewith, (c) and (d) are the correct answers.

Other references:
A. M. Sayeed& B. Aazhang, ‘Joint Multipath-Doppler Diversity in Mobile Wireless Communications’, IEEE Trans. on Commun., vol. 47, no. 1, 1999, pp 123-132.