All vertebrates are chordates but all chordates are not vertebrates, why?

The correct option is A Notochord is replaced by vertebral column in adult of some chordates
All chordates are characterized by the presence of a rod-like structure on the dorsal side, the notochord. It is present in all the chordates (urochordates, cephalochordates and vertebrates) during their embryonic stages. Hence, all vertebrates are chordates as they possess notochord during embryonic stages of their lives.

In cephalochordates, notochord is present throughout life. However, in urochordates notochord is found only in the tail of larva and in the vertebrates, it is replaced by the vertebral column in adults. All vertebrates possess vertebral column.

Therefore, all chordates are not vertebrates as urochordates and cephalochordates lack the vertebral column.