Questions: The sum of all costs that change as output changes divided by the number of units produced: Marginal cost Average variable cost Average (total) cost

The answer is the second one: Average variable cost.

Explanation for each option:

1. Marginal cost: This is the additional cost incurred by producing one more unit of a good or service. It is not the sum of all costs that change as output changes divided by the number of units produced.

2. Average variable cost: This is the sum of all variable costs (costs that change with the level of output) divided by the number of units produced. This matches the description given in the question.

3. Average (total) cost: This is the sum of all costs (both fixed and variable) divided by the number of units produced. It includes fixed costs, which do not change with the level of output, so it does not match the description given in the question.

Summary: The sum of all costs that change as output changes divided by the number of units produced is the Average Variable Cost.