A substance, such as aun, that is produced by a plant and affects how the plant grows and develops is best called a

The given question is incorrect or misspelled, the correct question is:

A substance, such as auxin, that is produced by a plant and affects how the plant grows and develops is best called a(n).

The correct answer is a plant hormone.

Explanation:

A hormone generated at the tip of the stem of the plant, which encourages cell elongation is known as auxin. Thus, auxin is a plant hormone that gets shift towards the plant's darker side and makes the cells to develop in a bigger size in comparison to the cells found in the plant's lighter end. This hormone is also accountable for the plant to curve in response to light, the phenomenon is known as phototropism.  

Apart from this one of the major functions of the plant hormone auxin is to sustain apical dominance.