B. Write four equivalent fractions for each by multiplying the numerators and denominators.

$$\frac{1}{3} = \frac{1 \times 2}{3 \times 2} = \frac{2}{6} = \frac{1 \times 3}{3 \times 3} = \frac{3}{9} = \frac{1 \times 5}{3 \times 5} = \frac{5}{15} = \frac{1 \times 7}{3 \times 7} = \frac{7}{21}$$

अतः, $\frac{1}{3}$ के चार समतुल्य भिन्न हैं: $\boxed{\frac{2}{6}, \frac{3}{9}, \frac{5}{15}, \frac{7}{21}}$

$$\frac{1}{4} = \frac{1 \times 2}{4 \times 2} = \frac{2}{8} = \frac{1 \times 3}{4 \times 3} = \frac{3}{12} = \frac{1 \times 5}{4 \times 5} = \frac{5}{20} = \frac{1 \times 7}{4 \times 7} = \frac{7}{28}$$

अतः, $\frac{1}{4}$ के चार समतुल्य भिन्न हैं: $\boxed{\frac{2}{8}, \frac{3}{12}, \frac{5}{20}, \frac{7}{28}}$

$$\frac{1}{5} = \frac{1 \times 2}{5 \times 2} = \frac{2}{10} = \frac{1 \times 3}{5 \times 3} = \frac{3}{15} = \frac{1 \times 5}{5 \times 5} = \frac{5}{25} = \frac{1 \times 7}{5 \times 7} = \frac{7}{35}$$

अतः, $\frac{1}{5}$ के चार समतुल्य भिन्न हैं: $\boxed{\frac{2}{10}, \frac{3}{15}, \frac{5}{25}, \frac{7}{35}}$

$$\frac{1}{7} = \frac{1 \times 2}{7 \times 2} = \frac{2}{14} = \frac{1 \times 3}{7 \times 3} = \frac{3}{21} = \frac{1 \times 5}{7 \times 5} = \frac{5}{35} = \frac{1 \times 7}{7 \times 7} = \frac{7}{49}$$

अतः, $\frac{1}{7}$ के चार समतुल्य भिन्न हैं: $\boxed{\frac{2}{14}, \frac{3}{21}, \frac{5}{35}, \frac{7}{49}}$