The most trusted source of expectations is

b. a consumer's own experience

Explanation:

When buying a certain product/service, costumers rely on certain expectations that guide them through the process of choosing.

Naturally, the most trusted source of expectations is a consumer's own experience. Empiricism has reasonable arguments for this marketing management application of experience. We always tend to trust our own past experiences, rather than an experience or review of somebody else, no matter how close that person is.