Complete with who followed by the singular simple-past be-passive of promote. Of all the assistants, Nia was the only one ___ in June.
Ճիշտ պատասխան
who was promoted
Բացատրություն
The clause identifies Nia as the unique promoted assistant, so who points to the only one and takes singular was. Using were would shift agreement toward assistants and would not match the explicitly singular interpretation.