Choice "D" is correct. When a firm is unable to change suppliers easily, that is a factor that increases the bargaining power of the suppliers.
Choices "c", "b", and "a" are incorrect because they all are factors that increase the bargaining power of the customer, which are:
Customers make up a large volume of a firm's business.
There is much information available to customers.
The buyers have low switching costs.
There are a high number of alternate suppliers.
