。
She picked out the most expensive dress.
Literal
She [topic-は] number-one expensive dress [object-を] pick-out-did.
選び出す is a compound verb of selection: 選ぶ ('choose') + 出す ('bring out, produce') — the ~出す suffix here adds a sense of 'finding / extracting from a set,' parallel to 思い出す ('recall, bring back to mind') and 探し出す ('search out / find'). 一番高価 is the standard superlative: 一番 ('number one / most') + な-adjective 高価 ('expensive'). 高価 is a Sino-Japanese alternative to the more common (高い 'expensive/high'); it feels weightier, suited to luxury contexts like 高価な宝石 ('expensive jewels').