The notochord is present in the embryonic stage of the vertebrates. It is a flexible, rod - like structure of mesodermal cells. In later vertebrates development , it becomes part of the vertebral column and helps in muscle attachment.
Therefore, the correct answer is option 'A'.

Introduction:
The notochord is a flexible rod-like structure that is present in the embryos of all chordates. It plays a crucial role in the development of the vertebrate body and serves as a support structure. In this question, we are asked to identify in which group of organisms the notochord is present during the embryonic stage.

Explanation:
The notochord is a defining characteristic of chordates, which is a phylum that includes animals such as fish, amphibians, reptiles, birds, and mammals. Chordates share several key features, including a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.

Let's analyze each option to determine the correct answer:

a) All chordates:
This option states that the notochord is present in all chordates during the embryonic stage. This is the correct answer because the notochord is indeed present in all chordates, including both vertebrates (animals with a backbone) and non-vertebrates (animals without a backbone).

b) Some chordates:
This option suggests that the notochord is only present in some chordates during the embryonic stage. However, this is incorrect because the notochord is a universal feature of all chordates.

c) Vertebrates:
This option states that the notochord is present only in vertebrates during the embryonic stage. This is incorrect because the notochord is present in both vertebrates and non-vertebrates.

d) Non-chordates:
This option suggests that the notochord is not present in chordates but is present in non-chordates during the embryonic stage. This is incorrect because the notochord is a unique feature of chordates and is not present in non-chordates.

Therefore, the correct answer is option 'A' - all chordates. The notochord is present in the embryonic stage of all chordates, regardless of whether they are vertebrates or non-vertebrates.