Phases of the moon occur because
(a) We can see only that part of the moon which reflects light towards us.
(b) Our distance from the moon keeps changing.
(c) The shadow of the Earth covers only a part of the moon’s surface.
(d) The thickness of the moon’s atmosphere is not constant.

Hint: The lunar phase or Moon phase is the shape of the directly sunlit portion of the Moon as viewed from Earth. The lunar phases gradually change over the period of a synodic month (about 29.53 days), as the orbital positions of the Moon around Earth and of Earth around the Sun shift The phases of the Moon are the different ways the Moon looks from Earth over about a month. As the Moon orbits around the Earth, the half of the Moon that faces the Sun will be lit up. The different shapes of the lit portion of the Moon that can be seen from Earth are known as phases of the Moon.

Complete step by step answer:
Moon is the only satellite of earth and is the fifth largest satellite in our solar system.
Moon revolves around the earth in its own orbit. Sun is the principal source of light, hence, the part of sunlight which gets reflected after falling on the moon is visible to us as phases.
So we can see only that part of the moon which reflects light towards us.

Hence the correct option is (a)

Note There are four principal lunar phases: new moon, first quarter, full moon, and last quarter (also known as third or final quarter, or toenail moon), when the Moon's ecliptic longitude is at an angle to the Sun (as viewed from Earth) of $0^\circ $, $90^\circ $, $180^\circ $, $270^\circ $ respectively. Each of these phases appear at slightly different times at different locations on Earth.