How Do You Know Which Group Is Activating and Deactivating?

Hint: We need to know that the groups mentioned refer to groups involved in electrophilic substitution reactions. Electrophilic substitution reactions occur when an electrophile displaces a functional group in a molecule, usually but not always a hydrogen atom. Aromatic compounds are known for their electrophilic aromatic substitution processes, which are a frequent technique of adding functional groups into benzene rings. Electrophilic substitution can also occur in aliphatic molecules.

Complete answer:
We need to know that in comparison to hydrogen, a group is activating that accelerates the rate of an electrophilic aromatic substitution process. When we replace a hydrogen on benzene with CH3, we enhance the rate of nitration. In comparison to hydrogen, a deactivating group slows down the pace of an electrophilic aromatic substitution process. When the trifluoromethyl group, $C{F_3}$ , is swapped for a hydrogen on benzene, the rate of nitration is dramatically reduced.
 Hence, when the reactivity of the electrophilic substitution process rises, the stability of the activating group rises. In a deactivating group, the group's stability declines, lowering the group's reactivity.

Note:
We must note that in the end, the concept of knowing which group is activating and deactivating is based on experimental response rate data. It doesn't explain why each group's rate increases or lowers. The terms "activating" and "deactivating" simply relate to how each substituent affects the rate in relation to $H$ .