Final answer:
In Biology, a rigid, nonflexible shape fitting the substrate exactly is known as a template, aligning with the 'lock and key' model of enzyme action.
Step-by-step explanation:
The question pertains to an aspect of Biology, specifically in relation to enzyme-substrate interaction. When an enzyme's active site is complementary to the specific substrate, and the substrate fits perfectly without the active site changing shape, this is referred to as the 'lock and key' model. The rigid, nonflexible shape that fits the substrate exactly is known as a template. However, the more accepted model today is the 'induced fit' model, where the enzyme can adjust its shape slightly to accommodate the substrate.