How many stairs?
You can use this to find the work
U
W=mgh
And the power by
P=W/T
The addition of 24 kJ of energy will allow all of the mercury and lead to change from solid to liquid. The temperature of each substance will also increase.
Carbon is the answer. IF oxygen were on the list it would also be correct but for this its Carbon<span />
In a closed system, energy in form of heat (work) can be exchanged but not matter.
The answer to your question is C.
Hope it helped!