Step 1: Recognize the function and its components.
We are given the function \( f(x) = |x| \sin |x| \). The absolute value function \( |x| \) implies that the function behaves differently for positive and negative values of \( x \). We must differentiate the function piecewise.
Step 2: Differentiate the function for \( x>0 \).
When \( x>0 \), we have:
\[
f(x) = x \sin x
\]
Using the product rule, the derivative is:
\[
f'(x) = \sin x + x \cos x
\]
Step 3: Differentiate the function for \( x<0 \).
When \( x<0 \), we have:
\[
f(x) = -x \sin(-x) = x \sin x
\]
Again, using the product rule, the derivative is:
\[
f'(x) = \sin x + x \cos x
\]
Step 4: Apply the absolute value.
The function is piecewise defined, and we need to adjust the derivative for each case. Therefore, for both \( x>0 \) and \( x<0 \), the derivative is the same:
\[
f'(x) = \sin x + x \cos x
\]
Step 5: Conclusion.
Thus, the derivative of \( f(x) = |x| \sin |x| \) is \( f'(x) = \sin x + x \cos x \), corresponding to option (A).