With reference to answer (a) - A radalt failure would put the TCAS into TA only mode.
By way of an example, when we do a cat 3 briefing on my type one of the items is a RAD ALT test. Push the button and note indicated rad alt of 40 feet and an autoland failure annunciation. However, if you glance at the TCAS it will be seen to have gone into TA only mode. Thus its best to have a glance at the TD for conflicts before doing the rad alt test.
For (b) in principle, IIRC, the RA fail flag means that the display in question can't show the RA, rather than TCAS can't generate an RA. I.E. you could have 1 display RA failing, but the other one working. IIRC, when you have a situation where there is no display capable of showing the target RA, OR no vertical speed information available anywhere in the aircraft (so you can't see if you have acheived the RA) then the syustem should go to TA only.
For (c) The phrase 'data tag' usually refers to the relative altitude / vertical speed arrow. Obviously this is absent when the intruder has no altitude reporting, but there is no specific flag that will appear somewhere else on the display. I.E. the answer is a distractor.
As C&B said, enhanced RA's are green banded for 2500 - 3000 fpm. Crap question really.
CPB