The correct answer is option 2, Bacteria.

When milk is left at room temperature, it undergoes a process called fermentation, which is responsible for converting the milk into curd. This fermentation process is caused by bacteria.

Bacteria are microorganisms that play a vital role in various biological processes. In the case of milk, certain types of bacteria, such as Lactobacillus and Streptococcus, are naturally present. These bacteria consume the lactose present in milk and produce lactic acid as a byproduct. The lactic acid lowers the pH of the milk, causing it to curdle and form curd.

It`s important to note that the bacteria responsible for this process are safe to consume and have been used for centuries in the production of cheese, yogurt, and other fermented dairy products. These bacteria also have probiotic properties, which can be beneficial for gut health.

In summary, it is bacteria, not fungi or viruses, that convert milk into curd through the process of fermentation.