Correct Answer: 38.9° C

Solution : The average temperature of Sunday and Monday $=37.2^\circ$
The sum of the temperature of Sunday and Monday $=2\times37.2^\circ=74.4^\circ$
The average temperature of Tuesday and Wednesday $=39.4^\circ$
The sum of the temperature of Tuesday and Wednesday $=2\times39.4^\circ=78.8^\circ$
The average temperature of Monday, Tuesday and Wednesday $=38.1^\circ$
The sum of the temperature of Monday, Tuesday and Wednesday $=3\times38.1^\circ=114.3^\circ$
The temperature on Sunday = (the sum of the temperatures of Sunday, Monday, Tuesday and Wednesday) – (the sum of the temperature on Monday, Tuesday and Wednesday)
$=74.4^\circ+78.8^\circ-114.3^\circ$
$=38.9^\circ $
Hence, the correct answer is 38.9° C.