2.
Bob Dylan's musical genres include folk, rock, roll, rhythm, blues, country and gospel.
3.
Replace and with a comma after each musical style, except the last and.
Bob Dylan's musical genres include folk, rock and roll, rhythm and blues, country and gospel.
4.
Replace and with a comma after each musical style, but keep the last and and place a comma before it.
Bob Dylan's musical genres include folk, rock and roll, rhythm and blues, country, and gospel.
GLOSSARY
genre (N) — style, category (often used in the arts)
seriated list — a list of multiple items, a "comma series".
vertical list — bullet / bulleted list.
The Oxford Comma — is a comma placed before the final and (or) in a list of items (The sandwiches are peanut butter and jelly, ham and cheese, and roast beef.) See style manual opinions.
Also see Comma Uses for an overview or summary.
IMAGE
Cabello, Alberto. "Bob Dylan." Flickr, 26 June 2010, https://www.flickr.com/photos/pixelillo/4937398975. Photographed at Azkena Rock Festival 2010. Cropped and resized. Licensed under CC BY 2.0.