Final answer:
When a company pays cash for equipment, it increases assets and decreases cash.
Step-by-step explanation:
When a company pays cash for equipment, it has two effects on the accounting equation. Firstly, it increases the company's assets because equipment is considered an asset. Secondly, it decreases the company's cash, which is also an asset. Therefore, the effect on the accounting equation is an increase in assets and a decrease in assets, resulting in option D) Increase assets and increase stockholders' equity.