Step 1: Understand the question.
We need the biggest religious event in Northeast India.
Step 2: The Ambubachi Mela.
This festival is held every year at the Kamakhya Temple in Guwahati, Assam. Kamakhya is a famous Shakti Peetha.
Step 3: Its scale.
Lakhs of devotees, sadhus and tourists gather here. It is often called the Mahakumbh of the East because of the huge crowd.
Step 4: Rule out others.
The Karbi festival and Ali-Ai-Ligang are cultural or farming festivals. Me-Dum-Me-Phi honours Ahom ancestors. None draw the massive religious gathering that Ambubachi does.
Step 5: Conclude.
The biggest religious event is the Ambubachi festival at Kamakhya temple.
Answer: Ambubachi festival at Kamakhya temple