What is added to another bill because it is unlikely to pass on its own?
a. a rider
b. a sneaker
c. a slider
d. a coattail

What we usually add to another bill because it's unlikely to pass on its own would be a rider - a.

When we add riders to bills, we're hoping that these riders will increase the likelihood of the bill being passed where all the parties involved would also be happy about the recent changes (Which came from the rider).