Sometimes it's easier to look at fractions in decimal form to tell which are greater or lower than 1/2.
1/2= 0.5
a) 2/5= 0.4 
lower
b) 4/7= 0.571 
greater
c) 4/9= 0.444 
lower
d) 5/11= 0.4545 
lower
e) 6/13= 0.4615 
lower
ANSWER: 
b) 4/7 is greater than 1/2
Hope this helps! :)