Which of the following equations is not correct?

The correct option is A $2\times 6=4\times 5$
Let’s check each equation one by one.

a.
$3\times 6=9\times 2$
$3\times 6=18$ and $9\times 2=18$
The product of the expressions on both sides is $18$. Therefore, this equation is correct.

b.
$2\times 6=4\times 5$
$2\times 6=12$ and $4\times 5=20$
The product of the expressions on both sides are not equal. Therefore, this equation is incorrect.

c.
$5\times 4=2\times 10$
$5\times 4=20$ and $2\times 10=20$
The product of the expressions on both sides is $20$. Therefore, this equation is correct.

d.
$6\times 6=9\times 4$
$6\times 6=36$ and $9\times 4=36$
The product of the expressions on both sides is $36$. Therefore, this equation is correct.