Answer : 4. "extend"
Explanation :

• "Extend" means to prolong or lengthen, and it is the most suitable term when referring to a deadline.
• "Increase" and "enlarge" are more commonly associated with quantity or size and are not typically used to describe extending a deadline.
• "Broaden" means to widen in scope, and it doesn't convey the idea of extending a specific time limit.