Which type of fungicide prevents pathogens from establishing by forming a barrier on a plant's surface?

A protective fungicide is designed to create a barrier on the surface of plants, which is effective in preventing the establishment of pathogens such as fungi. These fungicides work primarily by being applied to the surface of the plant, where they inhibit the germination of spores and block infection from occurring. This protective layer serves as a first line of defense against disease-causing organisms.

In contrast, systemic fungicides are absorbed by the plant and move internally to provide protection from pathogens that are already present or that penetrate into the plant tissues. Contact fungicides, while they may also work at the surface level, typically adhere to the plant but do not have the same barrier-forming capabilities as protective fungicides. Organic fungicides vary widely in their modes of action and may not specifically work by forming a protective barrier.

Thus, the emphasis on forming a barrier specifically to prevent pathogen establishment makes the protective fungicide the correct choice for this question.