Deflection in uniform electric field follows parabolic trajectory like projectile motion.

Steps:

• Electric force F = qE gives acceleration a = qE/m perpendicular to initial velocity v.
• Time to traverse field width L is t = L/v, unchanged for both particles due to same v.
• Deflection d = (1/2) a t² = (1/2) (qE/m) (L/v)².
• For second particle, a' = qE/(2m) = a/2, so d' = (1/2) (a/2) t² = d/2.

Why B is correct:

• Halved acceleration from doubled mass directly halves deflection via d = (1/2) a t² formula.

Why the others are wrong:

• A: Ignores that deflection scales linearly with acceleration, not quadratically inverse.
• C: Would require doubled acceleration, but mass doubles halves it.
• D: Implies quadrupled deflection, opposite to mass effect on acceleration.

Final answer: B