An agreement among railroad company owners to not let prices fall below a certain amount was called a
A. trust
B. monopoly
C. gauge
D. pool

B. monopoly

Explanation:

Horizontal monopoly agreement is a form of agreement in which the various firms or companies in the same competitive industry form and arrangement to destroy or reduce competition greatly.

Hence, in this situation, an agreement among railroad company owners to not let prices fall below a certain amount is a perfect example of a Horizontal Monopoly Agreement, which is a form of Monopoly.