The correct answer is: C. one
The elasticity of factor substitution is a measure of how easily one input can be substituted for another in the production process. In a Cobb-Douglas production function, the elasticity of factor substitution is equal to one. This means that the inputs can be substituted for each other in a perfectly elastic manner.
Option A is incorrect because the elasticity of factor substitution cannot be zero. This is because if the elasticity of factor substitution is zero, then the inputs cannot be substituted for each other at all.
Option B is incorrect because the elasticity of factor substitution cannot be infinity. This is because if the elasticity of factor substitution is infinity, then the inputs can be substituted for each other in an infinitely elastic manner. This is not possible in reality, as there are always some limits to how easily one input can be substituted for another.
Option D is incorrect because the elasticity of factor substitution can be greater than one. This is because if the elasticity of factor substitution is greater than one, then the inputs can be substituted for each other in a more than perfectly elastic manner. This means that the inputs can be substituted for each other more easily than in a Cobb-Douglas production function.