Which of the following numbers is in standard form?

1. $\dfrac{-12}{26}$

2. $\dfrac{-49}{70}$

3. $\dfrac{-9}{16}$

4. $\dfrac{28}{-105}$

A rational number $\dfrac{p}{q}$ is in standard form if $q \gt 0 \text{ and } p, q$ have no common divisor other than 1.

In Option 1, 12 and 26 have the common divisor 2, so $\dfrac{-12}{26} = \dfrac{-6}{13}$

In Option 2, 49 and 70 have the common divisor 7, so $\dfrac{-49}{70} = \dfrac{-7}{10}$

In Option 3, the denominator 16 is positive and the HCF of 9 and 16 is 1.

In Option 4, the denominator is negative, so $\dfrac{28}{-105} = \dfrac{-28}{105} = \dfrac{-4}{15}$

Hence, Option 3 is the correct option.