Final answer:
The penetration peg that fungi form to invade the host directly is called a haustorium.
Step-by-step explanation:
The name of the penetration peg fungi form to invade the host through direct penetration is B) Haustorium. Many parasitic fungi, including biotrophic fungi like powdery mildews and rusts, produce this specialized structure to penetrate the host cell wall and interact with the plant cell plasma membrane. This interaction facilitates the transfer of nutrients such as sugars and amino acids from the host to the fungus. Examples include the fungal pathogens that cause smut and Dutch elm disease, both of which similarly utilize haustoria to infect plant hosts.
An appressorium is a specialized structure formed by certain fungi, such as pathogenic fungi, to facilitate direct penetration of the host's tissues. It acts as an anchor and exerts mechanical pressure to breach the host's surface.