Both seems right Tough one. I went for A for two reasons, A is short and A denotes ongoing action which seems right in this context and parallel to nearing.

satishchandra wrote:On nearing the Earth, charged particles coming from the Sun tend to be deflected toward the North and South poles by the Earth's magnetic field.

(A) charged particles coming from the Sun tend to be deflected toward the North and South poles by the Earth's magnetic field
(B) charged particles that come from the Sun tend to be deflected toward the North and South poles by the Earth's magnetic field
(C) charged particles tend to be deflected as they come from the Sun toward the North and South poles by the Earth's magnetic field
(D) the Earth's magnetic field tends to deflect charged particles that come from the Sun toward the North and South poles
(E) the Earth's magnetic field tends to deflect toward the North and South poles charged particles coming from the Sun

How to choose between A and B

In D and E, on nearing the earth incorrectly modifies the Earth's magnetic field. The intended meaning of the sentence is that the CHARGED PARTICLES are nearing the earth. Eliminate D and E.

In C, by the Earth's magnetic field is too far from deflected, the verb to which it refers. A modifier should be as close as possible to what it modifies. Eliminate C.

Between A and B, each of which is grammatically correct:
A present participle (VERBing) implies CONTEMPORANEOUS action: one action happening at the same time as another action. Thus, the modifier in A (COMING from the sun) conveys the intended meaning better than does the modifier in B (that COME from the sun). Whereas the latter modifier could imply that the particles come from the sun and tend to be deflected at different times, the present participle in A makes it crystal clear that the particles are COMING from the sun AT THE SAME TIME as they TEND TO BE DEFLECTED. Eliminate B.

The correct answer is A.