The correct answer is D. Uncover or find a valuable line of argument or information.
The phrase “strike gold” means to make a great discovery or to achieve great success. In the context of the question, it is used to describe the difficulty of finding new and valuable information about Shakespeare, since so much work has already been done on him.
Option A, “hit a golden spot,” is a literal interpretation of the phrase “strike gold.” However, it does not make sense in the context of the question, since the speaker is not talking about finding a literal piece of gold.
Option B, “come across gold,” is a more figurative interpretation of the phrase “strike gold.” However, it still does not make sense in the context of the question, since the speaker is not talking about finding a literal piece of gold.
Option C, “come across the word ‘gold,'” is the least likely interpretation of the phrase “strike gold.” It is not a common idiom and does not make sense in the context of the question.
Therefore, the correct answer is D. Uncover or find a valuable line of argument or information.