The correct option is B Busy
Options A and B:
'Busy' means 'to be engaged in doing some work'. 'Busily' means 'to actively do some work'.
For the given sentence, the word 'busy' is more suitable in meaning: the gardeners were fully engaged in painting the roses red.
The other meaning 'the gardeners were actively painting the roses' is comparatively strange. Hence option B is correct and A is incorrect.
Options C and D:
'Buried' means 'to put or hide underground'. The word 'burying' means the same, it refers to a present continuous action of 'putting or hiding something underground'.
These words do not complete the sentence in meaning. Hence options C and D are incorrect.