The mutualistic association between certain fungi and roots of vascular plants are known as

The correct option is mycorrhizae.

Here is a step-by-step explanation of why this answer is correct:

1. Understanding the term: The term "mycorrhiza" (plural: mycorrhizae) literally translates to "fungus-root" (from the Greek words mykes meaning fungus and rhiza meaning root). It represents a symbiotic, mutualistic relationship where both the fungus and the plant host benefit from each other.
2. Mutual Benefit:
- Fungal Benefit: The plant shares photosynthetic products (carbohydrates/sugars) with the fungus to fuel its growth.
- Plant Benefit: The fungal hyphae branch out extensively into the surrounding soil, vastly increasing the surface area for absorption. This helps the plant take up water and essential nutrients, particularly phosphorus, which are otherwise difficult for the plant roots to access directly.
3. Evaluating the other options:
- Haustoria: These are specialized structure projections from parasitic fungi or plants that penetrate the host's tissues to absorb nutrients. Unlike mycorrhizae, this is a parasitic relationship, not a mutualistic one.
- Mycelium: This is the vegetative part of a fungus, consisting of a network of fine white filaments (hyphae). While it is a fungal structure, it does not specifically refer to the symbiotic association with plant roots.
- Rhizoids: These are simple, hair-like root structures found in non-vascular plants (like mosses and liverworts) and some fungi that serve to anchor the organism and absorb water/nutrients, but they are not a mutualistic association between fungi and vascular plant roots.