The correct option is D Countable Noun
'Flowers' is a common noun and it is a countable noun, indicated by the use of the plural form of 'flower' i.e 'flowers'. 'Flowers' is not an abstract noun (option A) as we can see, smell, touch them. Basically, we can experience them through our senses. In the given sentence, 'flowers' is not a collective noun, it's merely a plural one. Therefore, option D is incorrect.