Final answer:
The word 'ominous' in the context of the sentence indicates that the patient's irregular breathing is a significant and potentially dangerous sign, pointing to a serious health issue.
Step-by-step explanation:
In this context, the word "ominous" means something that is significantly important and dangerous. The nurse is indicating that the patient's irregular breathing is a bad sign, suggesting that it may be an indicator of a serious health issue. Thus, the correct answer is option b: Significantly important and dangerous.