The expression \((\sqrt{x}+1)^{9}\) suggests that a good substitution would involve \(\sqrt{x}+1\), because it appears as a composite function. Therefore, the correct choice for \(u\) is:
\[ u = \sqrt{x} + 1 \]
So, the correct answer is C. \( u=\sqrt{x}+1 \).
Show moreβ¦