Concept: A rak'ah is one complete unit of salat, and its starting action is what begins that unit of prayer.
Explanation:The Takbiratul-Ihram is the opening declaration of "Allahu Akbar" that commences the prayer.
It signifies the worshipper's entry into the state of salat and marks the start of the first rak'ah.
After this takbir, the worshipper recites Surah Al-Fatihah, then moves into ruku (bowing), rises to qiyam bi itidal (standing upright), and finally performs sujud (prostration).
Therefore, a rak'ah begins with the Takbiratul-Ihram, not with ruku, qiyam bi itidal, or sujud.
Answer:The correct option is B. Takbiratul-Ihram.