a- There are three things for which al-wudu is obligatory.

1- As-Salah (Prayers)

Allah (exalted be He) said: “O you who believe, when you intend to offer As-Salah (prayers), wash your faces and your hands up to the elbows, rub your heads, and wash your feet up to (and including) the ankles.” (Al-Ma’idan:6).

2- Circumambulation of Ka’bah.

This is because the Prophet ﷺ said to a menstruating woman: “Do not perform Tawaf until you have taken a purificatory bath.” [ narrated by Al-Bukhari.]

3- Touching the Noble Qur’an:

Allah the most Exalted says: “None touch it (The Noble Qur’an) except the purified”(Waqiah:79)
source: https://www.al-feqh.com/en/wudu-ablution