Introduction: The scientific approach to knowledge is empirical and is based on observations and experiments. Most of the observations are quantitative (taking measurements or quantifying something regarding the process) but some of them are qualitative (description of a process) also.
(a) The violent reaction witnessed by a chemist on mixing sodium with water is best classified as an observation. Hence, option (a) is correct.
Observation is generally regarded as an act to sense the external real objects.
Theory can be described as a scientific explanation of an observed activity or a phenomenon. It explains the observations or events that are based upon any proven hypothesis.
A law is a description of an observed phenomenon or activity. It is generally regarded as a statement of fact.
A hypothesis is a supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation.
In this case, there is a violent reaction that is observed. So, it is an observation.