Fresh parsley is very high in vitamin C, and is one of the best overall sources of vitamins and minerals of any garden plant.
Legumes like clover and Lucerne (called Alfalfa in
some countries) are a valuable food as a part of a varied diet, but should not
be the main food for Guinea Pigs. Here is Lucerne growing in a field.
If you use vitamin C drops, put them unto the animals’ food, not its water. Guinea Pigs vary a lot in the amount they drink and in some situations, they may drink very little. Also, the Chlorine or Chloramine in tap water destroys Vitamin C very quickly. Even in water without these powerful oxidising agents the vitamin is very unstable and can disappear quickly.
Lettuce is a food that some people say shouldn't be given to Guinea Pigs, but I find they enjoy an occasional feed of lettuce leaves. Lettuce loses its vitamin C very fast after harvest and may have already lost half its content of this vitamin within a few hours. Supermarket lettuce will nearly always be much older than that. The best way of getting letuce, both for your pets, and for yourself, is by growing it yourself and feeding it immediately after picking. Cos lettuce has the advantage that you can pick just a few outside leaves, using them fresh and leaving the plant still growing and producing more leaves for tomorrow.
Rabbit pellets are intended for Rabbits, not Guinea Pigs. Many types of rabbit pellet do not contain added vitamin C. In some cases, they are based on Lucerne (alfalfa) hay. While these pellets are not harmful to Guinea Pigs in small quantities and can be used as a high protein, high energy supplement, as part of a varied diet, they are unsuitable to use as the main food for Guinea Pigs.