The correct option is B but
The clauses in the given sentence, "He is a slow writer" and "his work is the best yet" are independent as they do not depend on each other for their complete meaning. To combine them as a sentence, a coordinating conjunction is required. The sentence has a contrast meaning "works slow" and "is the best", as slow speed is an undesirable quality and "his work is the best" is a good quality. 'But' is a coordinating conjunction that expresses a contrast between these two qualities, hence option A is correct. 'Slow' is an adjective as it modifies the noun 'speed', describing the speed as 'relatively taking longer time than expected'. It does not combine the sentences, hence it's not a conjunction and option B is incorrect. Similarly, 'best' is an adjective as it modifies the noun 'work' referring to the work as that of the highest quality. It does not connect the sentences, hence it's not a conjunction and option C is incorrect. 'He' is a pronoun, that refers to a man that's been mentioned before, as it's a pronoun and does not combine the sentences here, it's not a conjunction and option D is incorrect.