What causes the mucus plug to break?

The correct answer will be - widening of the cervical canal to prepare itself for delivery of the child.

Explanation:

The mucus plug is the collection of the thick and acidic substance called mucus produced by the cervix during pregnancy period. The mucus seals the cervical canal acting as a barrier to microorganism and unwanted infections to protect the uterus carrying a child.

The mucus plug breaks down when the cervical canal prepares itself for the delivery of the child. Thus, mucous breakdown acts as a precursor for the delivery of the child.

Thus, widening of the cervical canal to prepare itself for delivery of the child is the correct answer.