Heat from fuels is sometimes used to heat water and create steam. the steam expands, compared to liquid water, and may be used to push a piston or turn a generator. in this case, the heat from the fuel is being most directly a. used to warm the building. b. transformed into potential energy. c. wasted as steam. d. used to perform work.

In this case, the heat from the fuel is being most directly "used to perform work".

A fuel is any material that can be made to respond with different substances so it discharges energy as heat energy or to be utilized for work. The idea was initially connected exclusively to those materials fit for discharging chemical energy yet has since likewise been connected to different wellsprings of heat energy, for example, nuclear energy (by means of atomic parting and atomic combination).  

The heat energy discharged by responses of fuels is changed over into mechanical energy by means of a heat engine. Different occasions the heat itself is esteemed for warmth, cooking, or mechanical procedures, and additionally the enlightenment that accompanies combustion. Fuels are additionally utilized in the cells of life forms in a procedure known as cellular respiration, where natural particles are oxidized to discharge usable energy.

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."