The correct option is D Came across
The context refers to meeting someone at an unexpected time as one does not make a prior appointment to see someone when they are checking out of a hotel. Hence, D is correct as "to come across" is to meet, find, or encounter especially by chance.
A is incorrect as "to fall over" is tofall to the ground.
B is incorrect as "to look up" is to seek someone out.
C is incorrect as "to hang up' is to cut a telephone connection.