Answer
$a. 20^{\circ}~C$
Work Step by Step
In the Sahara Desert, the daytime temperature can reach over $40^{\circ}~C$. Clearly $40^{\circ}~C$, $60^{\circ}~C$, and $80^{\circ}~C$ are too hot for a thermostat setting in a home.
A temperature of $20^{\circ}~C$ is not hot, but it is about the temperature of a warm spring day. This would be a comfortable temperature for a home.
The best estimate for the temperature of a setting on a home thermostat is $20^{\circ}~C$.
