It's B : 5/9 = 0.555… .
To see why, let x = 0.555… . Then 10x = 5.555…, and
10x - x = 5.555… - 0.555…
9x = 5
x = 5/9
Why the other answers are wrong:
• It's not A because 3/10 = 0.3, while 1/3 = 0.333…
• It's not C because 9/100 = 0.09, while 1/11 = 0.090909… :
x = 0.090909…
100x = 9.090909…
100x - x = 9.090909… - 0.090909…
99x = 9
x = 9/99 = 1/11
• It's not D because 47/99 ≠ 0.475475475… :
x = 0.475475475…
1000x = 475.475475475…
1000x - x = 475.475475475… - 0.475475475…
999x = 475
x = 475/999 ≠ 47/99