Answer: The run-on sentence is C. Jack was trying out for the baseball team it was his first time ever playing the game.
Explanation: There should be a period at the end of the first sentence: Jack was trying out for the baseball team.
Then start the next sentence with a capital letter: It was his first time ever playing the game.
Since there is a contrast between the two ideas,-- You would expect someone trying out for the team to have had some prior playing experience!-- it would be appropriate to connect the sentences with a comma and the conjunction, but.
Jack was trying out for the baseball team, but it was his first time ever playing the game.
All the other examples are punctuated correctly, so they are not run-on sentences.