Which of the following is a sign of a suspicious letter or package?

A rigid or bulky shape can indicate that a letter or package may be suspicious because it deviates from the typical characteristics of mail. Most standard letters and small packages generally have a consistent, flat appearance. When an item is unusually shaped or feels rigid, it raises red flags that could suggest the presence of dangerous materials or devices inside.

This characteristic prompts individuals to exercise caution and consider further investigation, as it's important to ensure safety and security when handling potentially hazardous items. While the other options might seem unusual, a rigid or bulky structure is particularly noteworthy in the context of suspicious mail, as it can imply manipulation or concealment of harmful contents.