Which of the following is not a subphylum of Chordata?

Observe the following simplified scheme of the division $Vertebrata$ and choose the correct option that matches with the letters given in the boxes.
$Vertebrata$ (Division)
- $p$ (Lacks jaw)
- $q$ (Bears jaw)
- Super class
- $r$ (Bear fins)
- $s$ (Bear limbs)

The notochord is restricted to the tail region only in:

What do $a$ and $b$ represent in the given chart respectively?

In $Urochordata$,the notochord is found in:

The notochord is .........