A player is deemed offside when he interferes with an opponent. Which scenario qualifies for this ruling?

A player is ruled offside when he interferes with an opponent in a way that affects the play. Blocking an opponent's view of the ball is a clear example of such interference, as it prevents that opponent from effectively participating in the play. This can disrupt the opponent's decision-making and ability to respond to the situation, thus meeting the criteria for being offside.

The other scenarios do not necessarily indicate interference. Being in front of the last defender may place the player offside, but it does not inherently imply interference unless combined with an action that affects the play. Running toward the goal does not equate to interference unless it affects an opponent's ability to play the ball. Lastly, if a player is not involved in the play, they cannot be considered offside. Therefore, blocking an opponent's view directly qualifies as active interference, making it the correct choice.