Which of the following is an example of a simile?

A simile is a figure of speech that compares two different things using the words "like" or "as" to create a vivid image or description. In the given example, "He ran as fast as lightning," the comparison uses "as" to liken the person's speed to that of lightning, which emphasizes how fast he was running. This comparison helps to evoke a strong visual image and conveys a sense of immediacy and intensity related to the person's speed.

The other choices illustrate different literary devices: "Her smile was radiant" expresses a strong quality of the smile, but it doesn't compare it to something else. "The world is a stage" utilizes a metaphor to depict life as a performance, and "Time is a thief" also uses a metaphor to suggest that time stealthily takes away moments from life, but neither compares with "like" or "as." Thus, the use of "as" in the correct answer exemplifies a simile effectively.