。
She decided to marry Tom.
Literal
She [topic-は] Tom [with-と] marry [thing-こと] [to-に] decided.
結婚する takes と (with), not を (object) — you marry 'with' someone in Japanese, framing marriage as a two-party joint action rather than something done to a partner. ~ことにする ('decide to') nominalizes the verb with こと and binds the decision to it via に, literally 'decide upon the thing of marrying.' Compare ~ことになる (it ends up being decided), where the decision feels imposed rather than chosen.