What does this saying mean; "Don't trust everything you see even salt looks like sugar"?

Who said this in of pride & prejudice, and what type of irony does it show? “you mistake me, my dear. i have a high respect for your nerves. they are my old friends. i have heard you mention them with consideration these twenty years at least.” a.mrs. bennet: situational irony b.mr. bingley: dramatic irony c.elizabeth: proverbial irony d.mr. bennet: verbal irony