Which two signs should be interchanged in the following equation to make it correct?
$$6 + 7 \times 8 - 21 \div 7 = 47$$

By Trial and Error method,

Option A

$$6-7\times8+21\div7=47$$

$$6-7\times8+3=47$$

$$6-56+3=47$$

$$-47=47$$

Hence Option A is incorrect

Option B

$$6+7\div8-21\times7=47$$

$$6+\frac{7}{8}-21\times7=47$$

The value will be decimal after solving which is not possible

Option C

$$6+7\times8\div21-7=47$$

$$6+7\times\frac{8}{21}-7=47$$

$$6+\frac{8}{3}-7=47$$

The value will be decimal after solving which is not possible

Option D

$$6\times7+8-21\div7=47$$

$$6\times7+8-3=47$$

$$42+8-3=47$$

$$47=47$$

Hence, the correct answer is Option D