Answer:
C 5/13
Step-by-step explanation:
A fraction is written in reduced form if there are no common factors in the numerator and denominator. Since you know your multiplication tables, you can easily identify the common factors:
A 20 = 5·4, so 4 is a common factor 4/(4·5) = 1/5
B 6 = 2·3, 9 = 3·3, so 3 is a common factor (2·3)/(3·3) = 2/3
C 13 is not a multiple of the prime number 5, so there are no common factors. 5/13 is a reduced fraction
D 14 = 2·7, 21 = 3·7, so 7 is a common factor (2·7)/(3·7) = 2/3
