What does 'tribulation' refer to?

The term 'tribulation' specifically refers to a state of suffering, distress, or difficulty that one faces. It encapsulates the idea of undergoing hardship or a challenging experience, often implying a sense of struggle or adversity. This aligns perfectly with the first choice, describing it as a difficult experience or trouble.

In contrast, the other options represent positive or neutral experiences that do not align with the definition of 'tribulation.' A moment of joy, for instance, suggests happiness rather than difficulty, while unexpected success refers to achieving something favorable without prior anticipation. Lastly, describing something as a common occurrence suggests it is an ordinary or usual situation, which is not consistent with the significant challenges implied by 'tribulation.' Thus, the correct interpretation connects deeply with hardship or difficulty, which is why the first choice is the most accurate representation of the term.