What is the value of inventory destroyed?

The mark-up is 50% on cost. This means that Sales = Cost + (50% of Cost) = 1.5 $\times$ Cost.
We can rearrange this to find the Cost of Goods Sold:

$$\text{Cost of Goods Sold} = \frac{\text{Sales}}{1 + \text{Mark-up Percentage}}$$

$\text{Cost of Goods Sold} = \frac{\$21,000}{1 + 0.50}$ $\text{Cost of Goods Sold} = \frac{\$21,000}{1.5}$ $\text{Cost of Goods Sold} = \$14,000$

Goods available for sale is the sum of the opening inventory and purchases.

$$\text{Goods Available for Sale} = \text{Opening Inventory} + \text{Purchases}$$

$\text{Goods Available for Sale} = \$15,000 + \$18,000$ $\text{Goods Available for Sale} = \$33,000$

The value of the inventory destroyed is the closing inventory, which is calculated by subtracting the Cost of Goods Sold from the Goods Available for Sale.

$$\text{Inventory Destroyed} = \text{Goods Available for Sale} - \text{Cost of Goods Sold}$$

$\text{Inventory Destroyed} = \$33,000 - \$14,000$ $\text{Inventory Destroyed} = \$19,000$

The value of the inventory destroyed is $19,000. This corresponds to option C.

The final answer is $\text{C}$.