The correct answer is option (3), Observations
The scientific investigations basically start with an observation which is followed by thought provoking questions. For example – newton discovered gravity when he observed a falling apple. He realized that instead of falling in any other direction , an apple always fall towards the ground. This observation led to the discovery of gravity. There are several other observations made by scientist which led to great findings such as- Sky being blue in color, Appearance of moon during the nighttime only, etc