If "retort" means "to reply sharply or angrily," then its opposite could be:
* Agree: To express concurrence or harmony with what was said.
* Concede: To admit or acknowledge something to be true.
* Accept: To receive or take something without objection.
* Acknowledge: To recognize the existence or validity of something.
* Comply: To act in accordance with a request or command.
* Yield: To give way to pressure or force.
If "retort" means "a sharp or angry reply," then its opposite could be:
* Agreement: A statement expressing concurrence or harmony.
* Acknowledgement: A recognition of the truth or validity of something.
* Acceptance: A willingness to receive or take something without objection.
* Compliment: An expression of admiration or praise.
If "retort" means "to react chemically," then its opposite would be something that doesn't react chemically, such as:
* Inertia: A state of rest or motion with no change.
* Stability: A state of balance or equilibrium.
The best opposite for "retort" will depend on the specific context.