The correct option is A Common
The word 'rare' refers to something out of the ordinary or not found in large numbers.
Option A, common refers to something ordinary or found or done often.
Option B, dear is used as an affectionate or friendly form of address.
Option C, unusual refers to something not commonly occurring or done.
Option D, strange refers to something unusual or surprising.
Thus, according to the meaning, the word 'common' is the antonym of the word 'rare'.