C
That happened to me once, when I waitressed. An older man asked if I could stick my my finger in his tea to make it sweet. I just awkwardly laughed and said "Haha, ...no?...." I'm really, really bad at knowing how to respond to weird/forward stuff like that. Even if they're just joking.
It is seen as a challenge but needless to say I can repeat it so many times they get tired of hearing it,lol.