Km/h² or m/s/s are the appropriate units of the acceleration.
Acceleration is the rate change of velocity of an object. it is given by
a=ΔV/Δt
Δv= change in speed
Δt= time taken
Δv is measured in meter of km
Time is measured in second s or hour h
so the units of acceleration are km/h² or m/s/s