A chemical substance produced by a microorganism for inhibiting the growth of others is known as:

The correct option is A Antibiotic
Antibiotic is a chemical secreted by microorganisms (bacteria or fungi) that kills or hinders the growth of some kinds of bacteria.

For example:

1. Penicillins are a group of broadspectrum antibiotics originally obtained from Penicillium moulds.
2. Streptomycin is an antibiotic produced by gram-negative bacteria to treat several diseases.