Final answer:
The sale of a machine for less than its net book value leads to an increase in current assets, a decrease in equipment (net), and a decrease in net income.
Step-by-step explanation:
When a machine having a net book value of $15,000 is sold for $12,000, the correct answer is option A: current assets increase, equipment (net) decreases, and net income decreases. This transaction results in cash (a current asset) increasing by $12,000, which is the sale price of the machine. The equipment account on the balance sheet, which reflects net book value, decreases by $15,000, the original value of the machine. Lastly, there is a loss on the sale of the asset because the machine sold for less than its book value; hence, net income decreases by $3,000 ($15,000 - $12,000).