Right option is (b) 0.343403400…
Explanation: 1/3=0.3333… and 2/3=0.6666…0.333… and 0.555… are non-terminating and recurring, so they can be represented in the form p/q where p and q are integers. Hence they are rational numbers. 0.343403400 and 0.67670671… are non-terminating and non-recurring. Hence they are irrational numbers. But, among 0.343403400…and 0.67670671, only 0.343403400 is between 1/3 and 2/3. So option 0.343403400… is correct.