A

Step 1: Understand the nature of electric and magnetic fields.
Electric fields do work on charged particles, causing them to gain kinetic energy as they move within the field.
Step 2: Analyze the effect of magnetic fields.
Magnetic fields exert a force on moving charged particles, but this force (given by the Lorentz force law) is perpendicular to the velocity of the particle, thus not doing work on it. Therefore, while charged particles can change direction due to magnetic fields, they do not gain energy from magnetic fields.
Step 3: Conclusion.
Since only electric fields do work and allow the electron to gain energy, the correct answer is Option A: Electric field.