Which of the following gifts is most likely to be unethical?

Choose only one answer below.

a$.$ Juan, a chef and restaurant owner, receives a fruit basket from one of his grocery suppliers to celebrate the opening of his second restaurant location.

b$.$ A stationery manufacturer sends free pens and paper samples bearing its branding to other companies in the area.

c$.$ A company throwing a catered party for its staff gives the catering company a $25\%$ tip for their services.

d$.$ Erin, a video game critic, is offered an all$-$expenses$-$paid vacation by the publisher of an upcoming game she is planning to review.