Final answer:
The word 'abrasive' best completes the sentence, describing a person who is blunt and speaks their mind without regard for others' opinions.
Step-by-step explanation:
The sentence 'He is a very _____ person; he always speaks his mind, regardless of what others think.' can be completed with the word 'abrasive'. An abrasive person is someone who is forthright and candid, often to the point of being blunt or somewhat harsh, which can be seen as 'rubbing others the wrong way'. This characteristic aligns with the description of someone who 'always speaks his mind, regardless of what others think'. The other options provided - 'Nullify', 'Hasten', and 'Tangible' - do not fit the context of the sentence as they have different meanings. 'Nullify' means to make something invalid, 'Hasten' means to hurry, and 'Tangible' means perceptible by touch or clear and definite; real.