We are given the expression \( x^4 + 4x^2 + 3 \). Let's check if any of the options are factors of this expression.
Let's first factor \( x^4 + 4x^2 + 3 \):
\[
x^4 + 4x^2 + 3 = (x^2 + 3)(x^2 + 1)
\]
Thus, \( x^2 + 3 \) is a factor of the given expression. Therefore, the correct answer is \( x^2 + 3 \).