Given that a/b = -p/C = 1/q, make C the subject.

We have the equation:

$$\frac{a}{b} = \frac{1}{p} - \frac{1}{C}$$

To make $C$ the subject, first move the term $\frac{1}{C}$ to the left side and $\frac{a}{b}$ to the right side:

$$\frac{1}{C} = \frac{1}{p} - \frac{a}{b}$$

The common denominator for $p$ and $b$ is $pb$.

$$\frac{1}{C} = \frac{b}{pb} - \frac{ap}{pb}$$ $$\frac{1}{C} = \frac{b - ap}{pb}$$

$$C = \frac{pb}{b - ap}$$

Discrepancy with Options:
The derived answer is $C = \frac{pb}{b - ap}$. However, all the provided options (A, B, C, D) contain a variable 'q' which is not present in the original equation. This indicates a potential typo in either the question or the options.

Given the requirement to select an option, and assuming there is a typo where 'q' in the options should be 'p', let's re-evaluate. Even with this assumption, none of the options directly match $C = \frac{pb}{b - ap}$.

If we assume the question intended to be $\frac{a}{b} = \frac{1}{C} - \frac{1}{p}$ and 'q' in the options is a typo for 'p', then:

$$\frac{1}{C} = \frac{a}{b} + \frac{1}{p}$$ $$\frac{1}{C} = \frac{ap+b}{bp}$$ $$C = \frac{bp}{ap+b}$$

Comparing this with the options (assuming $q=p$):
A. $C = \frac{bp}{ap+bp} = \frac{bp}{p(a+b)} = \frac{b}{a+b}$ (Not a match)

Due to the clear inconsistency with the variable 'q' in the options, a definitive answer cannot be provided from the given choices based on the question as written. However, if forced to choose the closest form, and assuming a typo in the question where it should have been $\frac{a}{b} = \frac{1}{p} - \frac{1}{q}$ and the options are for $q$, then $q = \frac{pb}{b-ap}$. None of the options match this.

Given the constraints, I must select an option. This implies there is an intended correct answer among the options. Without further clarification on the typo, it's impossible to be certain. I will state the derived answer and highlight the discrepancy.

The mathematically derived answer from the given equation is $C = \frac{pb}{b - ap}$.
Since this does not match any of the options due to the presence of 'q' in the options, I cannot confidently select an option.