Which of the following would increase friction?
a. water
b. motor oil
c. sand
d. soap

Aside from sand, the rest is used to reduce friction between to objects, acting much more like a lubricant. This is on the context that the rougher the surface, the more friction is created. Then again, factors are to be considered especially as to what objects are in contact with each other when these options are added to the intention of increasing friction and finding out which doesn't do the trick. As for this case, since it there isn't any specifics and as to the kind of friction, then sand would be the likely answer.