A scalar quantity is one which has only a magnitude - by contrast, a vector quantity has magnitude and a direction. The existence of a direction, on a simplistic level, can be ascertained by considering whether the quantity would make sense with a negative sign on the front.
A negative acceleration is simply a deceleration, so this must be a vector quantity.
A negative displacement is a displacement backwards, so this is a vector too.
A negative velocity is a speed backwards, so this is vector also.
The remaining option is length - length has to be positive because it refers merely to the distance between two points. The length of a iPhone is the same whether it is measured from top to bottom, or from bottom to top (i.e. no negative sign is used).
Therefore, the scalar quantity is length.
I hope this helps you
