There exists a close association between the alga and the fungus within a lichen. The fungus:

In a lichen, the fungus provides protection, anchorage, and absorption for the alga. The fungus forms a symbiotic relationship with the alga, providing it with a safe and stable environment in which to grow. In return, the alga provides the fungus with nutrients through photosynthesis. So, the correct answer is option B. Notice that while the fungus does provide some benefits to the alga, it does not fix atmospheric nitrogen, provide food, or release oxygen specifically for the alga. Therefore, options A, C, and D are incorrect.