Assertion: Some vertebrate embryos develop a series of six pair aortic arches.
Reason: These aortic arches were gradually dropped out during the development.
A)Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B)Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C)Assertion is correct but Reason is incorrect.
D)Both Assertion and Reason are incorrect.

Hint: The aortic arch is the part of the principal artery that twists between the ascending and descending aorta. It leaves the heart and rises, at that point dives back to make the arch.

Complete answer:
The aorta receives oxygenated blood from the left ventricle of the heart and distributes it to the other parts of the body. Aortic arches are a progression of six paired embryological vascular structures which offer ascent to the arteries of the neck and head. They are ventral to the dorsal aorta and emerge from the aortic sac. Aortic arches are also called pharyngeal arch veins and branchial curves in human embryos.

Option A: There are six paired aortic arches present in the form of embryological vascular structures. Thus, Both the Assertion and Reason are correct but the Reason is not the correct explanation for Assertion. Therefore, this is the incorrect option.

Option B: Although the six pairs of aortic are not present simultaneously, they develop and regress at different stages of development. Thus, both the Assertion and Reason are correct and Reason is not the correct explanation of the Assertion. Therefore, this is the correct option.

Option C: The six pairs of aortic arches gradually drop out during the development. Assertion and Reason both are correct. Therefore, this is the incorrect option.

Option D: Both statements are correct. Therefore, this is the incorrect option.

Hence, the correct answer is option (B)

Note: The defects of the great arteries arise as a result of irregular existence of regressed aortic arches or regression of aortic arches that normally should be present. The ventral aorta extension remains as the external carotid in all groups.