The correct answer is C. Increase in stock.
A cash outflow is an expenditure of cash. Option A, increase in creditors, is a cash outflow because it represents an increase in the amount of money that a company owes to its suppliers. Option B, increase in debtors, is also a cash outflow because it represents an increase in the amount of money that a company is owed by its customers. Option D, increase in bills receivable, is also a cash outflow because it represents an increase in the amount of money that a company is owed by its customers for goods or services that have been delivered but not yet paid for. Option C, increase in stock, is not a cash outflow because it represents an increase in the amount of inventory that a company has on hand.
Increasing stock does not require a company to spend any cash. In fact, it can actually lead to a decrease in cash if the company has to pay for the inventory with cash. Therefore, increase in stock is not a cash outflow.