Which of the following is classified as a "condition of obligation" (Shart al-Wujūb) for Wudu rather than a condition of validity?

Wudu: Conditions of Obligation and Validity

• Encompassing the entire skin with purifying water.
• Reaching the age of puberty (al-Bulūgh).
• Removing a solid wax barrier that prevents water reaching the skin.
• Scrubbing the toes three times.