To answer this question, we need to have knowledge of the characters involved in the battle of Kurukshetra and their actions.
Option A) Sahdev - This option is incorrect. Sahdev was one of the five Pandava brothers, but he did not possess the knowledge of entering a chakravyuh or battle formation based on encirclement.
Option B) Ekalavya - This option is incorrect. Ekalavya was a talented archer, but he did not possess the knowledge of entering or getting out of a chakravyuh.
Option C) Abhimanyu - This option is correct. Abhimanyu, the son of Arjuna and Subhadra, possessed the knowledge of entering a chakravyuh but did not know how to get out of it. He fought bravely in the battle of Kurukshetra and was eventually killed when the Kauravas broke his bow and attacked him from behind.
Option D) Karna - This option is incorrect. Karna was a skilled warrior and a key character in the Mahabharata, but he was not the brave boy who knew the secret of entering a chakravyuh.
Therefore, the correct answer is Option C) Abhimanyu.