Question

a naturally produced plant growth stimulant has been found in ______.
a. hardwood mulch
b. corn
c. the soil
d. alfalfa

Explanation:

To solve this, we recall plant growth stimulants. Alfalfa (d) is known to naturally produce plant growth stimulants (like auxins, gibberellins - related compounds). Hardwood mulch (a) is for mulching, corn (b) has different uses, soil (c) contains some but alfalfa is a plant source of such stimulants.

Answer:

d. alfalfa