Chassaignac's tubercle lies at the level of -
Erb's point is opposite chassaignac's tubercle (tubercle on transverse process of C6 vertebra), 2-3 cm above the clavicle, slightly outside the lateral border of sternocleidomastoid.
Chassaignac's tubercle is carotid tubercle on C6 vertebra, and it lies at the level of Erb's point.
Option 'b' also requires some explanation here. Stellate ganglion block is given most commonly by using chassaignac's tubercle as a landmark. But stellate ganglion itself lies at the level of the transverse process of C7 vertebra.
Erb's point
Stellate ganglion
Atlas
Odontoid process