Which of the following transactions increases both assets and liabilities?

The question asks which transaction increases both assets and liabilities. Let's analyze each option:

• A. Buying equipment on credit: When equipment is bought, the asset "Equipment" increases. Since it's bought on credit, a liability like "Accounts Payable" also increases because the company owes money. This transaction increases both assets and liabilities.
• B. Writing off a bad debt: This reduces "Accounts Receivable" (an asset) and "Allowance for Doubtful Accounts" (a contra-asset), so it affects assets but not liabilities.
• C. Owner's capital contribution: This increases "Cash" (an asset) and "Owner's Equity," but it does not affect liabilities.
• D. Paying salaries in cash: This decreases "Cash" (an asset) and increases "Salaries Expense," which reduces "Owner's Equity." It does not increase assets or liabilities.

Therefore, option A is the correct choice.

The final answer is $\boxed{\text{A}}$