The correct answer is $\boxed{\text{coulomb}}$.
A coulomb is the unit of electric charge in the International System of Units (SI). It is defined as the charge transported by a constant current of one ampere in one second.
The force between two point charges is given by Coulomb’s law:
$$F = \frac{k q_1 q_2}{r^2}$$
where $k$ is Coulomb’s constant, $q_1$ and $q_2$ are the charges of the two points, and $r$ is the distance between them.
In this case, the force between the two charges is $9 \times 10^3$ N. If the charges are equal, then each charge must have a charge of $q = \sqrt{k F r^2} = \sqrt{(9 \times 10^9 \text{ N m}^2/\text{C}^2)(9 \times 10^3 \text{ N})(1 \text{ m})^2} = 1 \text{ C}$.
The other options are all smaller units of charge. A milli-coulomb is one thousandth of a coulomb, a micro-coulomb is one millionth of a coulomb, and a pico-coulomb is one trillionth of a coulomb.