The correct answer is: C. prevent sulphation.
Trickle charging is a method of charging a battery at a low current for a long period of time. This helps to prevent the build-up of lead sulfate on the plates of the battery, which can lead to sulfation and a loss of capacity.
Option A is incorrect because trickle charging does not help to maintain proper electrolyte level. The electrolyte level should be checked regularly and topped up if necessary.
Option B is incorrect because trickle charging does not increase the reserve capacity of a battery. The reserve capacity is the amount of time that a battery can provide a certain amount of current before it discharges completely.
Option D is incorrect because trickle charging does not keep a battery fresh and fully charged. A battery should be fully charged regularly, and trickle charging is not a substitute for a full charge.