Which red is more suitable for creating a warm red effect?

Correct answer: Orange-biased red

The governing concept is warm–cool bias on the colour wheel. An orange-biased red leans toward yellow, and yellow and orange are conventionally associated with warmth. It will therefore create a warmer red effect than a violet- or blue-biased red, which leans toward the cool side. A greyed red mainly has reduced chroma, so it may look dull rather than strongly warm. Hence, option B is the most suitable choice.