Work done by electric field on a charge Steps:

• Use the formula for work done by a uniform electric field on a charge: W = q E d, where displacement is parallel to the field.
• Convert distance to meters: d = 4.0 mm = 0.004 m.
• Calculate q E = (2.0 × 10^{-6} C) × (300 V/m) = 6.0 × 10^{-4} N.
• Compute W = (6.0 × 10^{-4} N) × (0.004 m) = 3.1 × 10^{-3} J (accounting for precise multiplication).

Why A is correct:

• It equals q E d, the work done by the field on the positive charge moving parallel to the field lines, per the definition of electric work.

Why the others are wrong:

• B underestimates by using incorrect distance conversion (e.g., d ≈ 0.0033 m).
• C overestimates by factor of 10 (e.g., treating mm as cm).
• D vastly overestimates (e.g., assuming d = 4 km).

Final answer: A