'One third' as a decimal is 1 ÷ 3 = 0.333333..... ⇒ The decimal is recurring
'One fifth' as decimal is 1 ÷ 5 = 0.2 ⇒ The decimal terminates at the tenth value
'One seventh' as decimal 1 ÷ 7 = 0.142857142857.... ⇒ The decimal is recurring for every digit between 1 and 7
'One ninth' as decimal 1 ÷ 9 = 0.111111.... ⇒ The decimal is recurring
Answer: one fifth