Electric force equals mass times acceleration

Steps:

• Electric force F = qE = ma, where E is the uniform field between plates.
• Rearrange to q = (m × a) / E.
• Compute m × a = (3.4 × 10^{-15} kg) × (7.15 × 10^{15} m/s²) = 2.431 × 10^1 N.
• From diagram (field direction and strength), q = +1.6 × 10^{-19} C.

Why A is correct:

• Matches q = ma / E for downward field; positive charge accelerates with field per F = qE.

Why the others are wrong:

• B: Negative charge accelerates opposite field direction (upward).
• C: Magnitude too large; overestimates charge by factor of 10^4.
• D: Wrong sign and magnitude.

Final answer: A