Recall the formula for relative humidity.

Relative humidity ($RH$) is defined as the ratio of the actual vapour pressure to the saturation vapour pressure, expressed as a percentage.

$$RH = \frac{\text{Actual Vapour Pressure}}{\text{Saturation Vapour Pressure}} \times 100\%$$

Given:

• Actual Vapour Pressure = $12 \text{ mmHg}$
• Saturation Vapour Pressure = $16 \text{ mmHg}$

$$RH = \frac{12 \text{ mmHg}}{16 \text{ mmHg}} \times 100\%$$ $$RH = 0.75 \times 100\%$$ $$RH = 75\%$$

The correct option is C.

$$\boxed{\text{C. } 75\%}$$

Dalton's Law of Partial Pressures states that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases.

The correct option is B.

$$\boxed{\text{B. Sum of partial pressures}}$$