The correct answer is B.
Marginal cost is the change in total cost that results from producing one additional unit of output. It is calculated by taking the difference between the total cost of producing $n$ units of output and the total cost of producing $n-1$ units of output.
In mathematical terms, marginal cost is given by the following equation:
$$MC = T{C_n} – T{C_{\left( {n – 1} \right)}}$$
where $MC$ is marginal cost, $TC_n$ is the total cost of producing $n$ units of output, and $TC_{\left( {n – 1} \right)}$ is the total cost of producing $n-1$ units of output.
Option A is incorrect because it calculates marginal cost by taking the difference between the total cost of producing $n-1$ units of output and the total cost of producing $n$ units of output. This is not the correct way to calculate marginal cost.
Option C is incorrect because it calculates marginal cost by dividing total cost by the number of units of output produced. This is not the correct way to calculate marginal cost.
Option D is incorrect because it is not one of the possible answers.