TEM - Threat and Error Management - is a strategy to detect threats, errors and undesired aircraft states and apply a countermeasure to prevent an escalation of the situation, ultimately preventing an accident or incident.

SOPs are a common way to detect issues. They must be followed as written, and this will minimise errors. If an SOP is not followed correctly, or in this case not followed at all, it is the job of the other pilot to spot this and raise the issue. (Multi crew is a good countermeasure in itself as two pilots are more likely to spot each other's mistakes.)

Wait until a safe phase in the flight to discuss the procedure deviation. After an uneventful landing, report this to the company's safety department --> INCORRECT. This is too late - what if the pilot has already forgotten something important such as putting the gear down?

Request a level-off from the Air Traffic Controller and challenge the Commander about the procedure deviation --> INCORRECT. This is too drastic - firstly you would not disrupt the progress of the flight to point out a deviation from SOPs (this has not - yet - caused a critical safety situation, although it must be addressed immediately) and secondly "challenging" the Commander is not constructive and might lead to conflict and distraction.

Notice the procedure deviation and insist on using the company's checklist to be sure that all items are checked --> CORRECT. 1) the co-pilot has caught the deviation and 2) he is "insisting" on following SOPs. Note the language - he is being assertive, not aggressive. Professional pilots know that SOPs are to be followed, and they should not object to a deviation being pointed out to them.

Ignore the procedure deviation to maintain a good cockpit environment. The aircraft lands uneventfully. --> INCORRECT. This is too passive. How do you know the aircraft will land uneventfully? Maintaining a good cockpit environment should never be prioritised over a safety issue. Following good communication strategies (assertive, fact based, not afraid to speak up) will ensure that any mistakes or deviations are handled correctly and will not disrupt the environment. Humans make mistakes and we all need to accept having them pointed out in a reasonable way.