Which sentence uses the underlined word correctly?

I am no genius but I'm pretty sure its the first choice.

The research assignment came with explicit instructions on source citation.

Explanation:

The word 'explicit' means that something has been expressed without vagueness or ambiguity: it is fully clear and leaves no question about its meaning or sense.

The second option does not make sense at all due to the contradiction presented between the word 'explicit' and the phrase 'difficult to understand'. Something explicit is coherent and understandable for all people. It is almost the same case with the third one: if something is explicit, then there should not be a variety of interpretations because the sense and meaning were already stated and cleared.

The fourth option has the word 'implicit' in there and it is not correct either, because of the sentence sense and the meaning of the word. 'Implicit' is the opposite of 'explicit': something implicit is not directly expressed but understandable at some point from something else. The sentence clearly expresses a situation where the message has to be explicit, accurate and clear for those who read it in order to do something (instructions, in this case.)

Therefore, the first option is the correct answer. The underlined word and the sentence coincide in sense and meaning.