。
She claimed to be the owner of the land.
Literal
She [topic-は] that land [genitive-の] owner [copula+quotative-だと] insisted.
~と主張する ('to insist that, to claim that') uses the quotative と to embed the asserted content — here, that she is the owner. 所有者 ('owner, proprietor') is a formal term, distinct from the casual 持ち主 — common in legal, real-estate, and bureaucratic Japanese. The whole sentence has a courtroom or land-dispute flavor, the kind of phrasing you'd see in a news article about a property feud. 主張 carries an assertive, 'pushing' quality — you 'press' your claim with this verb, not just utter it.