Which of Newton's Laws explains why an object originally moving in a circular path will continue in a straight line path if the centripetal force is reduced to zero?

Newton's 1st Law

Newton'' 2nd Law

Newton's 3rd Law

Newton's Law of Gravitation

When object is moving in circular path then if suddenly its force reduced to zero then it will move to straight line

This is defined by Newton's First Law

So as per first law of Newton we can say

When there is no net force on an object then it will continue its state of motion in its original state without any change in it.

This is also known as law of inertia

so correct answer will be

Newton's 1st law