The temperature rise after neutralisation
One calorie is the amount of energy (heat) needed to raise 1 gram of water 1 degree Celsius at a pressure of one atmosphere. (It is almost constant to 3 significant figures, but there is some temperature dependence...) Also assume that 1 cm^3 of water is 1 gram which is more than good enough for the calculations needed for the problem. 

So for the reaction in the problem given, 50 mL of water by 6 degrees is 300 calories.

For the other reactions I'll give calories and temperature rise for product solution. 

(a) 600 calories, 12 degrees C

(b) 600 calories, 6 degrees C

(c) 300 calories, 3 degrees C

(d) 300 calories, 1.5 degrees C