Final answer:
An inertial reference frame is a frame of reference in which Newton's first law holds true. The only option that represents an inertial reference frame is the stationary train.
Step-by-step explanation:
An inertial reference frame is a frame of reference in which Newton's first law holds true - an object at rest remains at rest, and an object in motion continues in motion with constant velocity, unless acted upon by an external force. Based on this definition, the only option that represents an inertial reference frame is the stationary train.