Diameter of the earth as seen from the moon is about 2°.
(b) At earth-moon distance, moon is seen as (1/2)° diameter and earth is seen as 2° diameter. Hence, diameter of earth is 4 times the diameter of moon.
Dearth/Dmoon = 4
(c) Rsun/rmoon = 400
(Here r stands for distance, and D for diameter.)
Sun and moon both appear to be of the same angular diameter as seen from the earth.
(Dsun/Dmoon) = 400
But Dearth/Dmoon = 4
Dsun/Dearth = 100.