The singular subject is referred to the subject which is singular in number. The verb used with them is singular. If the noun used after the preposition 'of' is singular, the verb used is also singular. In the fourth sentence, the noun used after 'of' is 'fruit'. 'Fruit' is singular in number and it also uses the singular verb 'looks'. All the other three sentences have plural subjects.
I feel like yes because having students volunteer makes it easier for them to have an opportunity to be leaders and to be able to make a difference in their community, while volunteering also improves he or shes leadership skills.