Unique to primary growth is the fact that it
a. increases the thickness of the stems.
b. lengthens the height of the stems.
c. occurs in the cork cambium.
d. takes place in the vascular cambium.

The correct answer is B. Lengthens the height of the stems.

Primary growth is responsible for increasing the length of the plant both above and below the ground. In other words, primary growth increases the plant height. The plant height is elongated due to the shoot apical meristem (a tissue of active cell division present at tip of plant stem).

On the other hand, secondary growth corresponds to increase in the girth of the plant. It occurs due to secondary meristem ( that is cork cambium and vascular cambium).