The correct answer is option 'C' - Skylark.

Explanation:
The song "Skylark" is a popular American jazz standard written by Hoagy Carmichael and Johnny Mercer. It has been performed by numerous artists over the years, including Ella Fitzgerald, Frank Sinatra, and Sarah Vaughan. The song is about a person's longing for a carefree and joyful existence, represented by the skylark bird.

In the given question, the school boy is singing along with the skylark. This means that the school boy is joining in and singing along with the skylark as it sings its beautiful song. The skylark's song may inspire the school boy to sing along and enjoy the music.

It is important to note that the other options mentioned in the question do not fit the context of the given scenario.

- Option 'A' - His parents: While it is possible for the school boy's parents to sing along with him, the question specifically asks about who sings along with the school boy. There is no mention of the parents singing in this scenario.

- Option 'B' - Huntsman: A huntsman is a person who hunts animals, and there is no indication in the question that a huntsman is present or singing along with the school boy.

- Option 'D' - His friends: The question does not provide any information about the school boy's friends or whether they are present and singing along with him. Therefore, it cannot be assumed that his friends are the ones singing along with him.

In conclusion, based on the given information, the correct answer is option 'C' - Skylark. The school boy is singing along with the skylark.