Use the formula:
[tex] \frac{a}{b} \div \frac{c}{d} = \frac{a}{b} \times \frac{d}{c} = \boxed{\bf{\frac{ad}{bc}}}[/tex]
Let's solve your question:
[tex] \frac{6}{13}\div \frac{6}{12} = \frac{6}{13} \times \frac{12}{6} =\boxed{\bf\frac{12}{13}}}[/tex]
Your answer is C. 12/13.