The correct option is C White
The subject of a sentence is often modified by an adjective, or a word
or words that function as an adjective. They are called enlargement or
attributes.
In the given sentence, "White" is an adjective that modifies the subject "roses", hence it's an attribute and option A is correct. "Roses" is the subject, it cannot attribute itself; "Are" is the verb and "my favourite" is the object, hence options B, C and D are incorrect.