An enzyme that has become non functional due to the loss of its three-dimensional structure is said to be

Is said to be denatured.

Denaturation occurs when the environment around has a extremely high temperature of extreme pH values comparing to the optimum values of the enzyme. Each enzyme has their own optimum temperature and pH.

In the case, the lost its three-dimensional structure and can no longer function because the shape of the active site is altered and thus it can no longer bind into substrates.

The answer is option b.