What does materiality mean?

Which one of the following statements is correct concerning theconcept of materiality?

a. Materiality is determined by reference to guidelinesestablished by the AICPA.

b. Materiality depends only on the dollar amount of an itemrelative to other items in the financial statements.

c. Materiality depends on the nature of an item rather than thedollar amount.

d. Materiality is a matter of professional judgement.

How do you set limits on Materiality?