Final answer:
In autotrophic nutrition, such as photosynthesis, the arrow labeled 'a' would represent the direction of the movement of carbon dioxide, which is consumed by autotrophs to produce glucose. Option 2 is correct.
Step-by-step explanation:
The process of autotrophic nutrition, especially in the context of photosynthesis, involves several key players such as water, carbon dioxide, glucose, and oxygen.
When referring to the arrow labeled 'a' that would most likely represent the direction of the movement of a substance in autotrophic nutrition, we're looking at the inputs and outputs of photosynthesis.
The correct substance in this case would be carbon dioxide. During photosynthesis, autotrophs such as plants absorb carbon dioxide from the air. Using the energy from sunlight, they convert water and carbon dioxide into glucose and release oxygen as a byproduct.
This process is summarized by the chemical equation: 6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂. Therefore, the arrow labeled 'a' in the context of the photosynthesis diagram would represent the movement of carbon dioxide into the plant where it will be used to produce glucose.