Answer:
fixed frame of reference on Earth
Step-by-step explanation:
When speaking in daily life or in a written problem, in general we assume a fixed frame of reference on Earth, so that the speeds of objects are referred to Earth.
In some cases we talk about other frames of reference, but these are explicitly written.
It should be mentioned that all the reference frames are inertial, that is, with zero acceleration, in order to comply with Newton's second law