Choice C is the best answer. Paragraph 3("that...appreciation") suggest that gift-givers assume a correlation between the cost of a gift and how well-received it will be: "... gift-givers equate how much they spend with how much recipients will appreciate the gift (the more expensive the gift, the stronger a gift-recipient's feelings of appreciation)." However, the authors suggest this assumption may be incorrect or "unfounded" (line 47,"assumption...unfounded."), as gift-recipients "may not construe smaller and larger gifts as representing smaller and larger signals of thoughtfulness and consideration”. (lines 63-65,"As...onsideration.")
Choices A, B, and D are all incorrect because the passage neither states nor implies that the gift-givers' assumption is insincere, unreasonable, or substantiated.