Which of these entities is an indicator of the SO2 pollution of air?

The correct option is Lichens.

Step-by-Step Explanation:

1. Understanding Air Pollution Indicators (Bioindicators):
Bioindicators are living organisms (such as plants, planktons, animals, and microbes) that are utilized to screen the health of the natural ecosystem in the environment. They are highly sensitive to changes in their environment, and their presence, absence, or health status reveals the physical or chemical state of the surroundings.

2. Why Lichens are Sensitive to Sulfur Dioxide (SO₂):
Lichens are symbiotic associations consisting of a fungus and an alga living together. Unlike plants, lichens do not have roots, a protective waxy cuticle, or stomata to regulate water and gas exchange. Instead, they absorb water, nutrients, and gases directly from the atmosphere across their entire surface. This makes them highly susceptible to airborne pollutants, particularly sulfur dioxide (SO₂).

3. Mechanism of SO₂ Toxicity:
When lichens absorb sulfur dioxide from the air, the gas reacts with water inside the cells to form toxic sulfur compounds (like bisulfites and sulfites). These compounds disrupt vital metabolic processes, such as photosynthesis in the algal partner by breaking down chlorophyll. High levels of SO₂ lead to the decline, discoloration, and eventual death of the lichens. Consequently, in heavily industrialized or polluted urban areas with elevated SO₂ levels, lichens are noticeably absent, creating a "lichen desert".

4. Evaluating Other Options:

- Puffballs & Mushrooms: These are fruiting bodies of fungi. While fungi can accumulate heavy metals from the soil, they are not primary bioindicators of gaseous air pollutants like SO₂.
- Mosses: Mosses are bryophytes. Although some mosses are sensitive to pollution, lichens are globally recognized as the classic, most sensitive, and specific indicators of SO₂ pollution in the air.