Originally posted by Joe C.
I agree with survivor, i think it's C.
It's not really about The overall area of 'M' and 'W', look at the diameter of the tubes. This should clearly show that M is xylem tissue, as one of the mechanisms which carries water up the tubes is capillary action - the thinner the tubes are the better this works. Phloem tubes do not need to be as thin, as movement is by active transport.
Also, 'W', is an area of great variation as to tube diameters, this shows the larger tubes, which can carry the food up or down the plant due to active transport, much smaller tubes in amongst these are indicative of companion cells, which are necessary for the 'source to sink' mechanism.