Answer: d.Four-day walking journey from Mecca to Medina while reading from the Qur'an.
Step-by-step explanation:
From the above given options all of the actions are involved in the ritual of Hajj which Muslims have to perform once in their lifetime if they fall under the criteria of performing the Hajj.
They do undergo cleansing and wear white clothes.
They do stand on Mount Arafat for hours praying at the place where Muhammad (PBUH) held his last sermon.
They also definitely do the ritual of Stoning for the three consecutive days. BUT they do not go on a four day walking from Mecca to Medina reading the Holy Qur'an.