It can be CLEARLY apprehended that the BLANK requires an adjective to clearly explain the succeeding word. Since the options 1 and 4 are adverbs, they can be eliminated as an answer. 

‘Tacit’ and ‘taciturn’ are both adjectives but are different in meaning EVEN though they seem similar. ‘Tacit’ means something UNDERSTOOD without even being stated while ‘taciturn’ means inability of a person to speak. Here, according to the context, ’tacit’ is the most appropriate word and hence, the correct option.