If a crack grows from 1 inch to 1.5 inches within a day, this indicates that the crack has:

The correct choice is indicative of a process where the crack expands or increases in size over time, specifically in its length. When a crack grows from 1 inch to 1.5 inches, it shows a continuous and gradual increase, which is commonly referred to as propagation. The term "propagated" describes the behavior of the crack as it progresses through the material, demonstrating its movement and growth.

This concept is critical in visual testing because it provides insights into the integrity and stability of the structure being examined. Monitoring crack propagation helps assess potential failure points and understand the overall health of the material. Essentially, recognizing that a crack has propagated signifies that there is active growth which needs to be addressed before it leads to more serious structural issues.

In contrast, the other options do not accurately describe the phenomenon observed; "moved" does not clearly convey the change in size, "widened" implies an increase in thickness rather than length, and "exploded" suggests a sudden and violent failure, which is not the case with the gradual change noted in the question.