Assertion: Cephalization is first observed in Phylum Platyhelminthes.
Reason: Cephalization developed along with the evolution of bilateral symmetry.
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

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Hint: The Platyhelminthes, commonly known as flatworms, are a group of relatively simple bilaterian, unsegmented, soft-bodied invertebrates under phylum Platyhelminthes. Platyhelminthes were a primitive stage in the evolution of bilaterians. They have neither body cavity nor specialized circulatory and respiratory organs. This allows them to have a flat body and hence oxygen and nutrients diffuse through their bodies.

Complete answer:
Cephalization is the phenomena in which due to morphogenetic gradient the mouth, sense organs, and nerve ganglia are concentrated at the frontal end of the organism forming a head region.
This is associated with bilateral symmetry as well as movement.
Along with the evolution, cephalization has been altered generation after generation.
Cephalization was first observed in flatworms which are also known as Platyhelminthes. Flatworms of the phylum Platyhelminthes exhibit a little amount of cephalization and could not be completed due to the process of evolution.
Cephalization is a significant characteristic feature of the Bilaterian organisms. This group of organisms contains a large portion of animal phyla possessing the ability to move, using muscles.
Hence this can be concluded that both the Assertion and Reason are correct and the Reason is the correct explanation of Assertion.

Hence, the correct answer is option (A).

Note: If one studies the phylogenetic tree of the evolution of cephalized animals it can be observed that the group of animals showing high levels of cephalization are not closely related rather they are distinctly apart. Animals that catch prey or evade its predators are quite highly cephalized.