Answer:
Hello! the efficiency of a thermal engine depends on how much of the initial input is transformed into work, Here 270°c is the input and 210°c is the output.
Then the efficiency is calculated as e = 1 - Tc/Th, where Th is the hot temperature (in this case 270°C) and Tc is the cold temperature (in this case 210°C)
First we need to write the temperatures in kelvin degrees, this can be done by adding 273.15 to the celsius scale:
Th = 270 + 273.15 = 543.15
Tc = 210 +273.15 = 483.15
then the efficiency is e = 1 - 543.15/483.15 = 0.11
if we write this as a percentage, we get an efficiency of 11%, then the fourth answer is the correct one.
