To which part of the cell theory did Pasteur make the biggest contribution

Louis Pasteur demonstrated that cells can only arise from pre-existing cells.

Explanation:

The contributions of Louis Pasteur are one of the most remarkable in the history of humanity. Before Pasteur performed the experiments that revolutionized the cell theory, most people believed in the theory of Spontaneous Generation, which stated living organisms could arise from non-living matter.

However, in the 18th century, Pasteur performed a series of experiments that disproved the theory of Spontaneous Generation. By using using covered and non-covered flasks with broth, he demonstrated that cells were only able to grow in the broth if it was not properly covered, that is, if the air was exposed. This illustrated that cells couldn't arise from non-living matter but the opposite: cells can only arise from pre-existing cells. This was an extremely significant contribution to the cell theory.

c) All cells come from pre-existing cells.