Which of the following is not considered a serious infraction warranting a 15-yard penalty?

The correct choice reflects that offside is categorized as a minor infraction, typically resulting in a five-yard penalty rather than a more severe 15-yard penalty. This rule is in place because offside is more about a violation of the pre-snap alignment rather than unsportsmanlike behavior or a serious physical safety concern, which are often associated with the higher penalties.

In contrast, conduct detrimental to the game, roughing the passer, and taunting an opponent are all considered serious infractions. Conduct detrimental to the game may include any actions that violate the spirit and integrity of football, often resulting in significant penalties. Roughing the passer involves unnecessary or excessive contact with the quarterback after they've thrown the ball, highlighting player safety and deserving a harsher penalty. Taunting an opponent shows unsportsmanlike conduct that can escalate tensions on the field and disrupt the game's integrity, thus also warranting a substantial penalty.

Overall, the nature of the infractions determines the severity of the penalties in football, and offside is an established minor violation within the rules.