Ben,
Q1, I'm not 100% sure, but I think it would be open to interpretation. Service ceiling will always be at the same density altitude, so if we're talking about density altitudes, b) is the correct answer.
However, if the temperature is decreased, all other things being equal, the density altitude at a given point will be lower. Or, to put it another way, a given density altutude will occur at a higher actual altitude - and also at a higher pressure (or indicated) altitude. So answer c) could also be correct.
Q2 is easy - the stick shaker is a stall warning device, used in aircraft where there is insufficient natural stall warning, or where electronic systems mean there is no direct "feel" in the control column from the control surfaces. Answer c)
FFF
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