What is a unit of acceleration?
a. m/s
b. m/s2
c. m2/s
d. kg-m/s2

A). m/s ... meters per second. 
                 That's a speed.

b). m/s² ... (meters per second) per second. 
                   Acceleration.

c). m²/s ... square meters per second.
                 Rate of change of area.
                 Meaningless unit, unless you happen to be
                     a painter of houses or a mower of lawns.

d). kg-m/s² ... (mass) x (an acceleration). 
                      This is a unit of force.  
                       In fact, it's the definition of "newton".