Plastic imitation opal was first marketed in which country?

The correct choice is Japan because this country is historically significant in the production and marketing of plastic imitation opal. The development of synthetic gemstones, including imitation opal, became prominent in Japan during the mid-20th century when advancements in plastic manufacturing and the desire to create more affordable alternatives to natural stones grew. Japanese artisans and manufacturers played a key role in popularizing these synthetic materials, which led to their widespread use in jewelry.

The other options, while they may have their own history in gem production or have created various types of imitations, do not have the same clear association with the early marketing of plastic imitation opal as Japan does.