The correct option is B Air from outside, while entering the flask carried small bacteria.
Lazzaro Spallanzani constructed his experiment by placing broth in each of two separate bottles, boiling the broth in both bottles, then sealing one bottle and leaving the other open. Days later, the unsealed bottle was teeming with small living things that he could observe more clearly with the newly invented microscope. The sealed bottle showed no signs of life. This certainly excluded spontaneous generation as a viable theory. Except, it was noted by scientists of the day that Spallanzani had deprived the closed bottle of air, and it was thought that air was necessary for spontaneous generation. So, Spallanzani's experiment proved air from outside while entering the flask carried small bacteria.
Therefore, the correct answer is option B.