If a healthy worshipper performs a voluntary (Nafal) prayer sitting without any physical excuse, what is the impact on their reward?

Voluntary Prayers: Postures, Sitting, and Riding Beasts

• The prayer is invalid and carries zero reward.
• They receive exactly half (1/2) of the reward of a standing worshipper.
• They receive the complete, undiminished reward.
• They receive double the reward for humility.