In this question are given four statements followed by five conclusions, one of which definitely does not logically follow (or is not a possibility of occurrence) from the given statements. That conclusion is your answer.  

(Note: You have to take the four given statements to be true).

 
STATEMENTS:


=> All references are mails.

=>All mentions are references.

=>All comments are mentions.

=>No mail is a declaration.

CONCLUSIONS:

1) No reference is a declaration.

2) All comments are mails.

3) No mention‘ is a declaration.

4) All declarations being comments is a possibility.

5) At least some mails are mentions.