If an object is not accelerating, then one knows for sure that it is
a. at rest
b. moving with a constant speed
c. slowing down
d. maintaining a constant velocity

"Zero acceleration" means "velocity" that's not changing.

Sadly, it seems that both of these words are almost always
misinterpreted and misunderstood. "Acceleration" does NOT
mean "speeding up", and "velocity" does NOT mean speed.

"Velocity" is the speed AND direction of motion.  So, for example,
'30 miles per hour north' and '30 miles per hour west' are the same
speed but different velocities.

"Acceleration" is ANY change in speed OR direction.  Consider three
different families in three different cars.  One car is speeding up, one is
slowing down, and the third is driving around a curve at a steady speed.
ALL three of the cars are accelerating.