Concept:The obligatory acts of Hajj exclude Talbiyyah.
Explanation:The obligatory acts of Hajj include Tawaf, standing at Arafah, and Sa'y.
Tawaf is the act of circling the Ka'bah.
Arafah means pausing at the plain of Arafah on the 9th of Dhul-Hijjah.
Sa'y is the walking between Safa and Marwah.
Talbiyyah is a devotional declaration recited during Ihram.
It is not counted among the obligatory rituals of Hajj.
Therefore, Talbiyyah is the act that is not obligatory.
Answer:C. Talbiyyah