The correct answer is option 2: Biological control of crop diseases. While mushroom cultivation has several benefits, such as biogas production, recycling agricultural wastes, and potentially preventing cancer, it is not commonly used for biological control of crop diseases. Biological control typically involves the use of beneficial organisms, such as insects or microorganisms, to control pests or diseases that affect crops. While mushrooms have certain antimicrobial properties and can be used to produce bioactive compounds that have potential medicinal properties, they are not typically used specifically for crop disease control. Instead, other methods, such as crop rotation, pest-resistant crop varieties, or the use of chemical pesticides, are more commonly employed for crop disease control.