。
She attended a lecture on social welfare.
Literal
She [topic-は] social-welfare [about-についての] lecture [to-に] attended.
~についての (about/concerning) modifies 講演 (lecture) — a lecture on the topic of social welfare. 出席する (to attend) is the formal verb for attending events, meetings, or classes. 社会福祉 (social welfare) is a common compound in Japanese public policy discourse.