Correct Option: D

obstruction (Noun) : the fact of trying to prevent from making progress
impediment (Noun) : something that delays or stops the progress of something; obstacle; a physical problem that makes it difficult to speak normally
clear (Noun/Verb) : no longer in danger or thought to be guilty; away from something ; to remove something from a place; easy to understand
ailment (Noun) : an illness that is not very serious
incapable (Adjective) : not able to do something well