What year saw the first publication of Isaac Newton's theory on the laws of gravity?

The year that marks the first publication of Isaac Newton's theory on the laws of gravity is indeed 1687. This is when he released his seminal work, "Philosophiæ Naturalis Principia Mathematica" (Mathematical Principles of Natural Philosophy), commonly known as the Principia. In this groundbreaking book, Newton laid down the foundation for classical mechanics, including the law of universal gravitation, which describes the gravitational attraction between masses. This publication was significant because it not only introduced the mathematics behind gravity but also provided a comprehensive framework for understanding motion, laying the groundwork for future scientific advancements.

The other choices represent years that do not correlate with the publication of his work on gravity. 1649 and 1620 fall far earlier than the actual publication year, while 1685 precedes it by two years, as the Principia was published in 1687. Therefore, recognizing 1687 as the pivotal moment when Newton's gravitational theory was introduced is essential for understanding the historical context and development of physics.