The correct statements are:
- The moon has less gravitational force than Earth because it is smaller and has less mass;
The Moon is smaller than the Earth, and also it has much less mass than the Earth, its mass actually being extremely small for its size, and that in turn results with a much lesser gravitational pull by the Moon.
- The Sun's gravitational pull is what keeps the planets in its solar system in orbit;
The Sun has a very large mass, much larger than the planets, so its gravitational pull is what keeps them all in the solar system and doesn't allow them to wander out into the open space.
- The Sun has more gravity than any planet in the solar system because it has the most mass;
The Sun has the most mass in the solar system by far. If all planets are combined together they will still have much lesser mass than the Sun on its own.