Cost price of 28 articles is equal to selling price of 21 articles, then percentage of profit is = ?

Answer: Option B

Cost price of 28 articles = Selling price of 21 articles
$$\frac{\text{CP}}{\text{SP}}=\frac{21}{28}=\frac{3}{4}$$
Profit = Selling price - Cost price = 4 - 3 = 1
$$\begin{array}{rl}& \text{Profit }\%=\frac{1}{3}\times 100\\ & =33\frac{1}{3}\%\end{array}$$