Answer :
To rewrite the equation
$$
4x^4 - 21x^2 + 20 = 0
$$
as a quadratic equation, we use the substitution
$$
u = x^2.
$$
Notice that when we substitute $u$ for $x^2$, the term $x^4$ becomes $u^2$. That is,
$$
x^4 = (x^2)^2 = u^2.
$$
Substituting these into the original equation gives
$$
4u^2 - 21u + 20 = 0.
$$
This equation is a standard quadratic in $u$. Therefore, the substitution that should be used is
$$
\boxed{u=x^2.}
$$
This corresponds to the first option.
$$
4x^4 - 21x^2 + 20 = 0
$$
as a quadratic equation, we use the substitution
$$
u = x^2.
$$
Notice that when we substitute $u$ for $x^2$, the term $x^4$ becomes $u^2$. That is,
$$
x^4 = (x^2)^2 = u^2.
$$
Substituting these into the original equation gives
$$
4u^2 - 21u + 20 = 0.
$$
This equation is a standard quadratic in $u$. Therefore, the substitution that should be used is
$$
\boxed{u=x^2.}
$$
This corresponds to the first option.
