What does the landlady want billy to do, because according to her, "it's the law of
the land? "
pay a tip at the end of the visit
sleep in the basement
take a selfie
sign the guest book

Landlady wanted Billy to sign the guestbook,

Option (d);

EXPLANATION:

The story 'The Landlady' by Roald Dahl is a story of a '17-year old boy', Billy Weaver, who arrives in the new city. He wanted to make a good name in the world of business.

He is welcomed by an old landlady. She showed Billy his room and then insisted he sign in the guestbook. Though he found that there were only two names in that guestbook but still he believed the landlady.

Also, the landlady conveys the statement that it is the law for everyone to sign the guestbook before entering into the room.