The king is likely to marry . . . . . . . . heiress. A. a B. an C. the D. no article

The correct answer is: no article.

The reason is that the heiress is not specific. There are many heiresses in the world, and the king is not married to any of them yet. Therefore, we use no article.

If the heiress was specific, we would use an article. For example, if the king was going to marry the heiress of France, we would say “The king is likely to marry the heiress of France.”

We would also use an article if the heiress was the only one in the world. For example, if the king was going to marry the only heiress in the world, we would say “The king is likely to marry an heiress.”

However, in this case, the heiress is not specific and there are many heiresses in the world. Therefore, we use no article.