Final answer:
Focus on online fashion blogs to find a relevant publisher for your jewelry line ad. Therefore, the correct option is C.
Step-by-step explanation:
To find a relevant publisher with space on their website for your ad for a new jewelry line, you should focus on (c) Online fashion blogs. Online fashion blogs often have dedicated space for ads related to the fashion industry, including jewelry. These blogs attract a specific audience interested in fashion and style, making it a suitable platform to promote your jewelry line.