1. b That couldn�t have been the day we went to Margate because it was raining that day, not sunny.
The reasoning tells why that day was not possibly the right one, so "couldn�t have been" expresses the impossibility.
2. Actually, none of the choices are correct. The correct option would be "have had to have". Reading this sentence repeatedly makes it even more confusing, so I re-worded it, to get "Had the electrician not come in time, we would have been forced to have last night�s party by candlelight."
The answer that is probably intended is
c Had the electrician not come in time, we would have had to have had last night�s party by candlelight.
a and b can�t be right, because they are in the present, and d is wrong for the same reason as c, plus the fact that there is no need for the continuous form.
3. d Water is a very scarce resource in the desert.
The other words don�t work in this sentence.
I hope this helps you
Bruce