To calculate the expected monthly online sales based on the advertising spend using the given model \( y = 128.9 + 171.5x \), we need to substitute \( x \) (the amount spent on online advertising) with $350.00.
Using the formula:
\[ y = 128.9 + 171.5 \cdot (350) \]
Now, calculate \( 171.5 \cdot 350 \):
\[ 171.5 \cdot 350 = 59,025 \]
Now, substitute this back into the equation:
\[ y = 128.9 + 59,025 \] \[ y = 59,153.9 \]
So, the expected monthly online sales if the company spends $350.00 on online advertising is $59,153.90.
None of the provided options match this result exactly, so if required to choose from the offerings, double-check on possible rounding or interpretation discrepancies in the problem context. However, according to the calculations based on the model, the correct answer is:
$59,153.90