Identify the correct word from the given options, which means the following:
A plant or animal living on another.
a)Parasite
b)Hydra
c)Creeper
d)Bush

Hint: To be able to distinguish between the given words, the meaning of each group of organisms must be known.

Complete step-by-step answer:

 A plant or animal living on another is a parasite. It is an organism that lives on or in an organism of another species, referred to as the host, from the body of which it obtains nutriment.
 Option a, “parasite” is an organism that lives on another organism, which is what the given sentence says. It is the correct option. Option b, “hydra” is a type of freshwater organisms. They are found in tropical and temperate regions, which has nothing to do with the given sentence, hence, option b is incorrect. Option c, “creeper” is a plant that climbs up any solid platform it finds, which is irrelevant to the given context, hence, option c is an incorrect option. Option d, “bush” is a shrub or a clump of them, hence it an incorrect option. Clearly, option a is the correct answer.

Note: The most important part of solving this question would be to know the meanings of the given sentence and the options that follow. Here, one must know the meaning of parasite and be able to figure out which one can be it.