Cross-pollination is preferred because:

The correct answer is: It creates variation.

Cross pollination is preferred because it reduces chances of similar genetic makeup and produces more variation in the species where as self pollination does not bring any variation in the plant. Cross-pollination is preferred because it brings about variation in species. Self-pollination does not bring about any variations. Variation brings new traits to the plant which may be advantageous to the plant. For example, it helps the new plant to defend against diseases.