(D) Shar
Stemming cuts off affixes to get a base/root form, which may not be a real word. For 'Sharing', stemming removes '-ing' to give Shar, not a valid word.
Stemming is a crude, rule-based process that simply chops off suffixes. It does not guarantee a meaningful word (unlike lemmatization, which gives 'Share'). 'Shar' is the classic stemmed output of 'Sharing'. Examiners expect you to distinguish stemming (may produce non-words) from lemmatization (always produces valid base words).