(A) : Electric and gravitational fields are acting along same line. When proton and $\alpha -$ particle are projected up vertically along that line, the time of flight is less for proton.
(R) : In the given electric field acceleration of a charged particle is directly proportional to specific charge

(A) Electric and gravitational fields are similar to each other and the time of flight is less for the proton. So the assertion is true

(R) $a=\frac{qE}{m}$ 

$\frac{q}{m}$ is the specific charge of the charged particle. Therefore, acceleration is directly proportional to a  specific charge. So the reason is true

The correct option is A.