Final answer:
In badminton, if the birdie touches the line upon landing, it is considered in. The correct answer is a) In. Therefore, the correct answer to the question 'During game play, a player swings and misses the birdie, it hits the line, is it in or out?' is option a) In.
Step-by-step explanation:
When a player swings and misses the birdie, also known as a shuttle in badminton, and it lands on the line, the correct call is in. This aligns with the official badminton rules that stipulate the shuttle is considered in if any part of it touches the lines of the court. Therefore, the correct answer to the question 'During game play, a player swings and misses the birdie, it hits the line, is it in or out?' is option a) In.