。
She's majoring in economics.
Literal
She [topic-は] economics [object-を] is-majoring.
The suffix 学 ('study/discipline') turns a domain into an academic field: 経済 ('economy') + 学 = 'economics' as a subject of study, just as 歴史 + 学 = 'history (as a discipline)' and 言語 + 学 = 'linguistics.' 専攻する ('to major in') uses the resulting-state ~ている for an ongoing pursuit — Japanese treats a major as an active engagement one is currently in, whereas English allows either 'majoring' or 'majors' for the same idea. Universities ask 専攻は何ですか? ('what's your major?') as standard introductory small talk.