Explanation:
The correct sentence is option 'A': "He is a great guitarist."

Reasoning:
- In English grammar, when we talk about someone's profession or occupation, we usually use the indefinite article "a" before the noun.
- In this case, the noun is "guitarist," which refers to the person's occupation.
- The indefinite article "a" is used to indicate that the person is one of many great guitarists.
- The sentence "He is a great guitarist" means that he is skilled at playing the guitar, but he is not the only great guitarist in the world.

Examples:
- "He is a great singer."
- "She is a talented dancer."
- "They are skilled architects."

Incorrect options:
- Option 'B': "He is the great guitarist" is incorrect because it uses the definite article "the" instead of the indefinite article "a." The use of "the" suggests that he is the only great guitarist, which is unlikely.
- Option 'C': "He is great guitarist" is incorrect because it does not include an article before the noun "guitarist." The article "a" is necessary to indicate that he is one of many great guitarists.
- Option 'D': "None of these" is incorrect because option 'A' is the correct sentence.

Summary:
The correct sentence is: "He is a great guitarist." It follows the grammatical rule of using the indefinite article "a" before the noun "guitarist" to indicate that he is one of many great guitarists.