Q86

Can you choose not not use certain defenses at the time of an attack.

Ex.

I want to use hellish rebuke or armor of agathys or whatever and I am holding a shield. Can I choose to not add the bonus from the shield to my ac?

Or

If I have THP do I have to use them or can I choose to take the damage myself?

Or

Can I choose not to use arcane ward’s HP and instead take it myself to save the ho of the ward?