(A) : Spontaneous reaction may be slow or fast.
(R) : Spontaneous nature deals with feasibility of the reaction but not rate.

Both (A) and (R) are correct but (R) is not the correct explanation of (A).

Both (A) and (R) are correct and (R) is the correct explanation of (A).

(A) is correct but (R) is wrong.

(A) and (R) both are wrong.