Number of electrons = total charge / charge per electron Steps:

• Current I = V / R = 4.0 V / 20 Ω = 0.2 A
• Total charge Q = I × t = 0.2 A × 0.5 s = 0.1 C
• Charge per electron e = 1.6 × 10^{-19} C
• Number of electrons N = Q / e = 0.1 / 1.6 × 10^{-19} = 6.25 × 10^{17} Why C is correct:
• Matches the result from Q = I t and N = Q / e, using Ohm's law for current. Why the others are wrong:
• A: Wrong magnitude, possibly from unrelated constant like π in a misapplied formula.
• B: From omitting time, so Q = I = 0.2 C, N = 1.25 × 10^{18}.
• D: From underestimating charge (e.g., wrong t or V), yielding smaller N like 1.25 × 10^{17}.

Final answer: C