Shoot, I must have (must’ve) lost my key. Dang, I think I’ve lost my key. Crap, I don’t have my key. (Neither of your options sound right. How do you know you lost it? Oh my god, no) Darn it, I forgot my bag. Dang it, I’ve forgotten my bag. (Both sound fine, but “l’ve forgotten” is a little more upscale. Slight difference in meaning - focusing on past or present. )

I would always use the present perfect in this context. The present perfect , e.g. 'I've lost my key'*, is a present tense: it focuses us on the present moment and tells us something about the current situation. In this case, it tells us that that I don't have my key at the moment. This is distinct from the past simple, which tells us something about a past time: the past simple would tell us when or where or how you lost your key. For example "I lost my key (last night)" or "I lost my key (on the beach)". That said, many American English speakers will use the past simple in the contexts you describe. * Note that the auxiliary verb 'have' is nearly always reduced to 've' in spoken English statements. We never use the full form of the auxiliary 'have' in speech unless we want to add special emphasis e.g. to contradict a previous statement.

A is correct