Choose the correct form of the verb. Emily's mum ... flowers in the garden the day before yesterday.

The sentence is in the past simple tense, indicated by "the day before yesterday". For negative sentences in the past simple, the structure is "did not + base form of the verb". Therefore, "did not plant" is the correct form. The other options are incorrect because "did not planted" uses the past participle, "was not planted" is passive voice and doesn't fit the active subject "Emily's mum", and "was not plant" is grammatically incorrect.