Final answer:
Using the left-hand rule for negatively charged particles like electrons, we determine that the magnetic field is directed downwards because if the thumb points East (electron's direction) and the fingers point North (deflection), the palm points down.
Step-by-step explanation:
If an electron is moving horizontally to the East and is deflected North while in a vertical magnetic field, the direction of the magnetic field can be determined using the right-hand rule. For a negatively charged electron, the right-hand rule is inverted, meaning we use the left hand. Pointing the thumb of the left hand in the direction of the electron's velocity (East), and the fingers in the direction of the force experienced by the electron (North), the palm faces the direction of the magnetic field, which in this case is downwards. Therefore, the correct answer is (b) Down.