In British English we would almost certainly say "I haven't brought it." Because as it is important, it must be relevant to the present situation. The (lack of a) past action is connected to the present. We would probably say "I didn't bring [something]." if we weren't expected to bring it, as we wouldn't really be connecting the past to the present. But I agree with fdmaxey that in this case, the difference is small.

You would probably say "I didn't bring it." This would say that you forgot to bring it and there is little chance of bringing it now (though not impossible) "I haven't brought it" would be used more to mean "I haven't brought it yet.", implying that you might bring it in the future (sooner or later) It is a small difference.

Both would be acceptable. They both have the same meaning, but are constructed in a different tense.

I didin't bring it

Aha, thank so much u Wafiyah