The correct answer is A. preserve.
A blemish is a mark or imperfection that spoils the appearance of something. The opposite of a blemish is something that makes something look good or improves its appearance.
Preserve means to keep something safe from harm or decay. It can also mean to keep something in its original condition.
Purify means to make something clean or free from impurities.
Defect means a fault or imperfection.
Tarnish means to make something lose its brightness or luster.
In this case, the best answer is A. preserve. This is because preserve means to keep something safe from harm or decay, which is the opposite of a blemish, which is a mark or imperfection that spoils the appearance of something.