Write the Ratio as a Fraction in Simplest Form $\frac { 1 } { 3 }$

Write the ratio as a fraction in simplest form.
7 to 21

A) $\frac { 1 } { 3 }$
B) $3$
C) $\frac { 21 } { 3 }$
D) $\frac { 7 } { 3 }$
E) $\frac { 7 } { 21 }$