The sun is the primary gravitational force in the solar system because
a)it's mass and distance both increase gravitational force
b)it's nuclear energy is more powerful than any other in the solar system
c)it emits the most amount of heat at great distances
d)it's the most massive object in the solar system

The Sun is the primary gravitational force in the solar system
because it's the most massive object in the solar system.

It's estimated that the sun alone accounts for around 99% of the
total mass of the solar system. 

To put it another way, the sun, all by itself, has almost 100 times
as much mass as all of this solar system's planets, moons, comets,
dwarf planets, asteroids, and everything in the Kuiper Belt ... all put together !