The correct answer is 'sounding pleasant' (option B). EUPHONIOUS describes sounds that are pleasing to the ear, harmonious, or melodious. It comes from Greek 'eu' (good) + 'phone' (sound). 'Similar sounding words' is wrong (that would be homophones), 'a melodious sound/tone' is an example not a definition, and 'irritating sound' is the opposite (cacophonious).