Answer
$a. 10^{\circ}~C$
Work Step by Step
In the Sahara Desert, the daytime temperature can reach over $40^{\circ}~C$. Clearly $50^{\circ}~C$ and $70^{\circ}~C$ are too hot for a warm winter day in Washington, D.C. Also, $30^{\circ}~C$ could be the temperature of a hot summer day, so this temperature is also too hot.
A temperature of $10^{\circ}~C$ is not hot, but it is above the freezing point of water, so this seems like a reasonable estimate for the temperature on a warm winter day in Washington, D.C.
The best estimate for the temperature on a warm winter day in Washington, D.C. is $10^{\circ}~C$.
