A solid magenta line on a sectional chart, forming two concentric rings around a mid-size airport, indicates:

Why →Class C airspace on a sectional is depicted with solid magenta lines, typically showing two concentric rings (inner core from the surface to about 4,000 feet above the airport, and an outer shelf from about 1,200 feet to the same ceiling). Class D airspace is shown with a dashed blue line in a single boundary, and Class B is solid blue with multiple shelves around major airports. Color and line style together identify the class.

The trap →All three options describe rings around an airport, so the color and line style carry the whole answer: solid magenta = Class C, dashed blue = Class D, solid blue = Class B. Learning the ring pattern without the color is how this one gets missed.

Field note →The full key in one line: solid blue = B, dashed blue = D, solid magenta = C, dashed magenta = surface E.