An adjective is usually placed before the noun that it modifies. This is for an adjective that is in the attributive position, i.e., it comes before the noun. When an adjective is in the predicative position, i.e., when it follows the noun, the adjective does not follow it immediately; it follows a linking verb, thereby complementing the noun. (example: I am tall, I is the pronoun, 'am' is the linking verb and 'tall' is the adjective)
In the given sentence, the underlined word "oval" is an adjective; it follows the noun "mirror". (It cannot modify the verb that it precedes because adjectives don't modify verbs.) Hence option D is incorrect.
Option A is correct because the adjective is placed before the noun "mirror" that it modifies, it describes the shape of the mirror.
In option B, the adjective is placed before the noun "wall", but it doesn't make sense, walls cannot be oval, hence B is incorrect.
In option C, the adjective "oval" is placed before the noun "finish", which is abstract. Abstract nouns are ideas or states (finish means the manner in which an item is completed), they are not concrete, hence they do not have a shape (oval) and C is incorrect.