I dont know if this correct, but i hope it help
Answer: C2H2
Explanation: Because each of the lines represent one bond, and because there are three lines (bonds) between the carbons, it means that they are bonded by three bonds, also known as a triple bond.
To do this problem it is necessary to take into account that the heat given by the unknown substance is equal to the heat absorbed by the water, but considering the correct sign:

Clearing the specific heat of the unknown substance:
