答案：A

题目分析：句意：如果汤姆邀请你参加他的生日聚会，你最好给他买个礼物。you had better do sth,你最好做···，提建议或劝告。would like to do sth,想要做某事，be able to do 能够做某事，must do必须做某事。结合句意及结构，故选A。