By interchanging which two signs the equation will be correct?
$$6 \div 3 \times 5 - 15 + 4 = 9$$

Option (a) $$-\ and\ \div$$

$$6 - 3 \times 5 \div 15 + 4 = 9$$

$$6-3\times\frac{5}{15}+4=9$$

$$6-\frac{15}{15}+4=9$$

6-1+4=9

9=9

The given equation is satisfied. Hence this is the correct answer.

Option (b) $$\div\ and\ +$$

$$6 + 3 \times 5 - 15 \div 4 = 9$$

The given equation is not satisfied. Because it will give the values in decimal. Hence this is not the correct answer.

Option (c) $$\div\ and\ \times$$

$$6 \times 3 \div 5 - 15 + 4 = 9$$

The given equation is not satisfied. Because it will give the values in decimal. Hence this is not the correct answer.

Option (d) $$+\ and\ -$$

$$6 \div 3 \times 5 + 15 - 4 = 9$$

The given equation is not satisfied. Because it will give the values in decimal. Hence this is not the correct answer.