You can download the SOF NSO answer  keys from the official website i.e., www.sofworld.org (//www.sofworld.org) . If you can not find the link to download answer keys, here are the direct links to download the answer keys for each set of question paper:

Set A: http://www.sofworld.org/sof-nso-answer-keys-set-2019

Set B: http://www.sofworld.org/sof-nso-answer-keys-set-b-2019

Set C: http://www.sofworld.org/sof-nso-answer-keys-set-c-2019