The Summer is caused by

A. direct sunlight striking Earth

B. Earth's Closeness to the sun

C. Earth's far distance from the sun

D. Indirect sunlight striking Earth

A. Direct sunlight striking earth ... according to the lambert' cosine law, in winter cos (x) <1 which is responsible for low intensity of sunlight

It is summer in June in the Northern Hemisphere because the sun's rays hit that part of Earth more directly than at any other time of the year. It is winter in December in the Northern Hemisphere, because that is when it is the South Pole's turn to be tilted toward the sun.