To determine the selling price of the hybrid bike in September, we need to subtract $25 for each month from the original price of $399.
May: $399
June: $399 - $25 = $374
July: $374 - $25 = $349
August: $349 - $25 = $324
September: $324 - $25 = $299
Therefore, the bike shop was selling the hybrid bike for $299 in September.
The correct answer is $299.