The wavelength is b. 1.0m
One wavelength is 360 degrees. Or to simply it, check the attached image. A to E is one wavelength. F to J is one wavelength. At each node and antinode is 90 degrees. Or at each max and min and intersection with the flat line.
So if you start from the first node at the very left side and start counting, you’ll hit 360 at the middle. That’s one wavelength.