Although we can never truly know the correct answer, we can look back in the series and predict.
First off, we have a triangle (3 sides), then we have a rectangle (4 sides), and finally we have a pentagon (5 sides). The is a clear staple this series is making: it is adding sides to each shape as the series progresses.
Plus, the other answers have 4 and 8 sides, so I knew it wasn’t those.
