opt means to make a choice, especially of one thing or possibility instead of others
opt for means choose example (e.g. Mike opted for early retirement.)
opt in (into) means choose to be part or a member of something (e.g. Employees can choose to opt in of a lot of pension schemes.)
opt out means choose not to be part of something (e.g. The UK opted out of full EU legislations in accordance with the Brexit.)