The given sentence compares two things: the speaker's painting - "I made a painting...It was good" and the listener's painting - "as yours".
Option A: 'As' in 'as good' presents a comparison. It completes the sentence in meaning and grammar. Also, 'as...as' is a pair of conjunctions that is used in sentences in the positive degree. And 'good' is a positive adjective. Hence A is correct.
Options B, C and D:
'Good', 'better' and 'best' are the positive, comparative and superlative adjectives, respectively.
'Good' requires the conjunction 'as', 'better' requires the conjunction 'than' and 'best requires the article 'the', to form the suitable degrees of the sentence. Without these words, the sentence is grammatically and logically incorrect. Hence these options are wrong.