To simplify the expression \(415,000,000 + 558,000,000\), we start by performing the addition:
\[ 415,000,000 + 558,000,000 = 973,000,000 \]
Next, we will express \(973,000,000\) in scientific notation. We can rewrite \(973,000,000\) as:
\[ 973,000,000 = 9.73 \times 10^8 \]
Thus, the equivalent expression to \(415,000,000 + 558,000,000\) in scientific notation is:
\[ \boxed{9.73 \times 10^8} \]
This indeed corresponds to option D \(9.73 \times 10^8\).