The given food chain shows a sequence of organisms where each is a source of food for the next. To identify the missing link "A" in the food chain, we need to consider the dietary habits and ecological roles of the possible options. The food chain reads as follows:

• Plant → insect → frog → "A" → Eagle

Let's analyze the options:

• Option A: Wolf - Wolves are generally not part of a food chain that includes eagles because they are both top predators, and it is unlikely for a wolf to be preyed upon by an eagle.
• Option B: Rabbit - Rabbits primarily eat plants and would not be found in this part of the food chain after the frog. They are not typically a prey item for eagles after a frog in a direct food chain sequence.
• Option C: Cobra - Cobras can eat frogs, making them a suitable link in the food chain. In some ecosystems, large eagles, especially those adapted to hunting snakes like the Serpent Eagle, may prey on cobras.
• Option D: Parrot - Parrots are primarily herbivores or omnivores with a diet consisting mainly of seeds, nuts, and fruit. They are not typically part of a food chain where frogs are a preceding link and do not generally fall prey to eagles in the context of this type of food chain.

Considering the ecology of these animals, the most likely organism to fill the role of "A" between the frog and the eagle is the cobra, as it can be a natural predator to the frog and in turn can be prey for certain types of eagles.

Therefore, the correct answer is Option C: Cobra.