In this question, we are asked to identify an autotroph. An autotroph is an organism that can produce its own food using energy from sunlight or inorganic substances. Looking at the options provided, we can determine that a butterfly, grasshopper, and mushroom are not autotrophs because they cannot produce their own food.

Option 2, algae, is the correct answer because algae is an autotroph. Algae are photosynthetic organisms and can produce food through the process of photosynthesis, using sunlight and carbon dioxide.

Therefore, the correct answer is option 2, algae.