Which primary colour can be selected as a base colour for the sea or a clear sky?

Correct answer: Blue

The governing concept is the representational use of primary colours. Blue is a primary colour and is commonly chosen as a base colour for a clear sky or the sea because those subjects are conventionally associated with blue. Thus option B is correct. Red and yellow are primary colours but are less typical bases for these subjects, while orange is generally classified as a secondary colour rather than a primary one.