Ineed to know which piece of research in the square was done by john dalton.

C. atomic mass

Explanation:

John Dalton was a very versatile man. His work is important in physics, meteorology, medicine (description of color blindness) and chemistry. He was included in researching of partial pressures and atomic theory. He proposed one of the first models of atom in the beginning of 19th century.  

He published a table where he proposed relative atomic weight of six atoms. In his honor, standardized unit for atomic weight is called dalton (Da) and weighs 1/12 of carbon atom weight.