Explanation
The grammatically correct phrase to replace the bold part of the sentence is B: "Speaks for itself".
The original sentence is "I need not offer any explanation regarding this incident - my behavior is speaking itself". This sentence implies that the speaker's behavior is sufficient to explain the incident without requiring any further explanation.
The phrase "speaks for itself" means that something is so clear or obvious that it requires no further explanation. This phrase fits well with the meaning of the original sentence.
Option A "Will speak to itself" is incorrect because it implies that the behavior has not yet spoken and will do so in the future.
Option C "Speaks about itself" is incorrect because it implies that the behavior is discussing itself, which does not fit the intended meaning of the sentence.
Option D "Has been speaking to itself" is incorrect because it implies that the behavior has been speaking to itself over a period of time, which also does not fit the intended meaning of the sentence.
Therefore, the correct phrase to replace the bold part of the sentence is B: "Speaks for itself".
