The youngest secondary xylem occurs

Correct option is C

The correct answer is just inside the vascular cambium
The formation of the vascular cambium is initiated when cells between the columns of vascular tissue connect the cambia inside the columns of vascular tissue to form a complete cylinder around the stem. The cells formed towards the inside are called as secondary xylem or wood and those formed towards the outside of the cambium are known as secondary phloem. So, youngest layer of secondary xylem is located just inside vascular cambium.