Answer :
We start with the equation
$$
4x^4 - 21x^2 + 20 = 0.
$$
Notice that $x^4$ is the square of $x^2$. This observation suggests making the substitution
$$
u = x^2.
$$
Then, since $x^4 = (x^2)^2$, we can write $x^4 = u^2$. Substituting these into the equation, we obtain
$$
4u^2 - 21u + 20 = 0.
$$
This is now a quadratic equation in $u$. Therefore, the correct substitution is
$$
u = x^2.
$$
$$
4x^4 - 21x^2 + 20 = 0.
$$
Notice that $x^4$ is the square of $x^2$. This observation suggests making the substitution
$$
u = x^2.
$$
Then, since $x^4 = (x^2)^2$, we can write $x^4 = u^2$. Substituting these into the equation, we obtain
$$
4u^2 - 21u + 20 = 0.
$$
This is now a quadratic equation in $u$. Therefore, the correct substitution is
$$
u = x^2.
$$
