Sentence:  The differences of castes…furnish a copious stock of <WSD>extenuations</WSD>…to different classes of offences. 
Meaning of the word marked with <WSD> tags:  The action of (lessening), or seeking to lessen, the (guilt) of (an (offence) or (fault)) by (alleging) partial (excuses); and instance or (means) of doing this; a plea in mitigation of (censure).