Right answer is (c) θ = [(W+w)(t2-t1)]/m
To elaborate: Its unit is cal/gm. Here t2-t1 is the temperature difference and W+w is the total weight.
θ = [(W+w)(t2-t1)]/m
where θ is gross calorific value, W is the weight of the water sample taken in calorimeter, w is the weight of water equivalent, t1 is the initial temperature, t2 is the final temperature and m is the weight of the fuel sample taken respectively.