The U.S. Centers for Disease Control and Prevention claim that washing one's hands is most important step in preventing illness as it can ward off dangerous infections such as E. coli and salmonella.
THE most important step --> Eliminate AC
A. most important step in preventing illness as it can ward off dangerous infections such as E. coli and salmonella
C. a most important step in preventing illness as to do so can ward off dangerous infections such as E. coli and salmonella
"It" refers to noun, not a clause --> Eliminate B. ("Doing so" = washing one's hands)
B. the most important step in preventing illness as it can ward off dangerous infections such as E. coli and salmonella
Correct answer
D. the most important step in preventing illness as doing so can ward off E. coli, salmonella, and other dangerous infections
Which refers to samonella --> Eliminate E. A better way is "dangerous infections such as E. coli and salmonella"
E. the most important step in preventing illness as doing so can ward off infections such as E. coli and salmonella which are indeed dangerous