Which signs and numbers should be interchanged to make the following equation correct?
3 × 5 + 2 − 1 = 12 

Given: 3 × 5 + 2 − 1 = 12

Given equation is solve by BODMAS rule.

​$$\begin{array}{|c|c|} \hline\textbf{Operation preference wise} & \textbf{Symbol} \\\hline\text{Brackets} &[],{}, () \\ \hline \text{Orders, of} & (power), √ (root) , of \\ \hline \text{Division}& ÷ \\ \hline \text{Multiplication} & × \\ \hline \text{Addition} & + \\ \hline \text{Subtraction} & - \\\hline\end{array}$$​​​​

Now, we check each options.

Option (a): + and ×, 2 and 5

New equation: 3 + 2 × 5 − 1 = 12

3 + 10 − 1 = 12

13 - 1 = 12

Option (b): − and +, 5 and 2

​New equation: 3 × 2 - 5 + 1 = 12

6 - 5 + 1 = 12

2 $$\neq$$ 12​

Option (c): × and −, 2 and 5

​New equation: 3 - 2 + 5 × 1 = 12

3 - 2 + 5 = 12

6 $$\neq$$ 12​

Option (d): + and ×, 3 and 2

​New equation: 2 + 5 × 3 − 1 = 12

2 + 15 - 1 = 12

17 - 1 = 12

16 $$\neq$$ 12

Thus, correct option is (a).