The dominant period is about **900 nm**. The structures run roughly vertically in the image, with a slight tilt; their spacing is measured horizontally, perpendicular to the ridges. I used the **1 μm scale bar**, which spans 53 pixels (about 18.9 nm per pixel). The dominant spacing is about 47–48 pixels, or 890–910 nm.

RESULT: period = 900 nm