The correct answer is D. no article.
The phrase "a bird in the hand is worth two in the bush" means that it is better to have something that you are sure of than to hope for something that might be better. In this case, the "bird in the hand" is something that you already have, while the "two in the bush" are something that you might be able to get, but there is no guarantee.
The phrase does not use an article because it is not referring to a specific bird or bush. It is simply a general statement about the value of certainty over uncertainty.
The other options are incorrect because they all use articles. A and B use indefinite articles, which are used to refer to general or unspecified things. C uses the definite article, which is used to refer to specific things. In this case, the phrase is not referring to a specific bird or bush, so it does not need an article.
