#1: what did robert millikan's oil drop experiment prove?
a. atoms are composed of positive charges.
b. it determined the actual mass of the electron.
c. matter can be conserved.
d. atoms can be neither created nor destroyed.

Robert Millikan,in 1909,performed and experiment known as oil drop experiment.
During his experiment he made certain calculations and conclusions.One of them was that he determined the size of charge on an electron.He also proved that the charge was smallest 'unit' , or simply the charge is 'quantized'.
He was also awarded with Noble prize.
So,from the above explanation,i have concluded that the best option which refers to the question asked is :
B. It determined the actual mass of the electron.

Letter a i’d go with

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