A builder buys timber costing K800 at a discount of 12%. Calculate the amount the builder pays.

The original cost of the timber is K800.
The discount rate is 12%.

$$\text{Discount amount} = 12\% \times \text{K}800$$

Convert the percentage to a decimal: $12\% = \frac{12}{100} = 0.12$.

$$\text{Discount amount} = 0.12 \times \text{K}800$$ $$\text{Discount amount} = \text{K}96$$

The amount paid is the original cost minus the discount amount.

$$\text{Amount paid} = \text{Original cost} - \text{Discount amount}$$ $$\text{Amount paid} = \text{K}800 - \text{K}96$$ $$\text{Amount paid} = \text{K}704$$

The builder pays K704. This corresponds to option D.

The final answer is $\text{D. K704}$.