Final answer:
The electroscope may discharge if a bright light of sufficient frequency causes the photoelectric effect, which can expel electrons from the zinc plate. If the light's frequency isn't high enough, the electroscope retains its charge, making both options a and b possible, whereas c and d are not generally true.
Step-by-step explanation:
When a bright light is shone onto a zinc plate of an electroscope that is already charged, one of the effects is that the light can cause electrons to be emitted from the zinc surface if the light's frequency is high enough (photoelectric effect). This would result in the electroscope discharging. Therefore, statement (a) 'The electroscope discharges' can be true, especially if the electroscope was initially negatively charged. The emitted electrons would neutralize some of the positive charges. However, if the frequency of the light is not sufficient to cause the photoelectric effect, then no discharge would occur, and the electroscopeac would retain its charge, making statement (b) also a possibility.
As for the charge on the zinc plate, it would depend on the initial condition of the charge on the electroscope. The zinc plate does not become charged merely due to the bright light; the light can only cause the discharge of electrons if it is of sufficient frequency to cause the photoelectric effect. Therefore, statements (c) 'The zinc plate becomes negatively charged' and (d) 'The zinc plate becomes positively charged' are not generally true just because a bright light is shone onto the plate.