What is the name of the special hyphal tips through which parasitic fungi absorb nutrients directly from the cytoplasm of the living host?

The correct answer is Haustoria.

Parasitic fungi establish a specialized nutritional relationship with their host organisms. To extract nutrients directly from the cytoplasm of the living host cells without immediately killing them, these fungi develop specialized structures.
These structures are known as haustoria (singular: haustorium).

A haustorium is a specialized, simple or branched hyphal tip that penetrates the cell wall of the host. However, it does not rupture the host's plasma membrane (cell membrane). Instead, the host cell membrane invaginates around the growing haustorium, creating a large surface area for the efficient absorption of organic nutrients, water, and minerals directly from the host's cytoplasm.

Let's briefly look at why the other options are incorrect:
- Mildew refers to a type of plant disease caused by parasitic fungi (such as powdery mildew or downy mildew), or the growth of these fungi on surfaces, rather than the cellular structure itself.
- Constricting rings are specialized mechanical structures formed by certain predatory, nematode-trapping soil fungi to trap prey, not for direct nutrient absorption from living host tissues.