Which of the following equations will become mathematically correct after exchanging the operators according to the specified instructions: '+' stands for '-', '-' stands for '×', '×' stands for '÷', and '÷' stands for '+'?

The correct option is $18÷6+3=21$.

To determine which equation becomes correct, we need to replace the given operators according to the specified instructions:

• '+' stands for '-'
• '-' stands for '×'
• '×' stands for '÷'
• '÷' stands for '+'

Let's verify the correct option by substituting the operators into the expression on the left-hand side (LHS):

Given equation:

$18÷6+3=21$

Substitute '÷' with '+' and '+' with '-':

$\text{LHS}=18+6-3$

Now, perform the operations from left to right according to BODMAS rules:

First, add 18 and 6:

$18+6=24$

Next, subtract 3 from 24:

$24-3=21$

Since LHS = RHS ($21=21$), this equation becomes mathematically correct.