Choose the word which means the opposite of the underlined word in the sentence: Hydra is biologically believed to be immortal.
A) undying
B)perishable
C)ancient
D)eternal

Hint: Immortal means something that is not mortal, that is something that does not die.

Complete step-by-step answer:

From the hint given to us, we know that important means something that is not mortal and does not die. The opposite of immortal can be described as something that has an end to wit. Thus, we shall be analyzing all the words given to us in the options in order to find the correct opposite of immortal.
Undying means something that does not die. It is synonymous with immortal. Hence, option ‘a’ is not the correct answer; it does not match our description of the opposite word for immortal.
Perishable means something that has it into it. We observe that it matches our description for the opposite word of immortal. We can say that option ‘b’ is the is possibly a correct answer stop
Ancient means something that is very old in nature. We observed that it does not match our description for the opposite of immortal. Hence, option ‘c’ is not the correct answer.
Eternal means something that is long-lasting and something that has no end to it. It is synonymous with immortal. Hence, option ‘d’ is not the correct answer.

Note: Immortal can be used as a noun as well as an adjective. The same goes for perishable.