Which term best describes the cautionary nature of a 'portent'?

The term that best describes the cautionary nature of a 'portent' is indeed "ominous." A portent is often understood to be a sign or warning that something significant or negative is likely to happen. This inherently carries a sense of foreboding or dread, which aligns closely with the meaning of "ominous." When something is described as ominous, it suggests that there is an impending threat or danger, making it a fitting term to capture the cautionary aspect of a portent.

In contrast, the other terms do not align with this foreboding connotation. "Hopeful" conveys a sense of optimism and positivity, which contrasts sharply with the warning implied by a portent. "Indifferent" suggests a lack of concern or interest, which does not reflect the cautionary implications of a portent. "Common" relates to frequency or ordinariness and is not connected to the sense of warning or caution inherent in portents. Thus, "ominous" is the most appropriate choice to describe the nature of a portent.