To determine the expected monthly online sales based on the amount spent on online advertising using the given model \( y = 128.9 + 171.5x \), we need to substitute \( x \) with the amount the company spends on online advertising.
Here, \( x = 350 \) (the amount spent on online advertising):
\[ y = 128.9 + 171.5(350) \]
Now, calculate \( 171.5 \times 350 \):
\[ 171.5 \times 350 = 59,025 \]
Next, add this to 128.9:
\[ y = 128.9 + 59,025 = 59,153.9 \]
Thus, the expected monthly online sales if the company spends $350 on online advertising is:
\[ y = 59,153.9 \]
None of the provided options exactly match, but if we round this value, the closest option to \( 59,153.9 \) is \( $60,025.00 \).
Therefore, the answer is: $60,025.00.