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She fried the fish in salad oil.
Literal
She [topic-は] fish [object-を] salad-oil [with-で] deep-fried.
サラダ油 is a small false friend for English speakers: it doesn't mean an oil specifically for salads, but rather a category of refined, neutral-tasting plant oil — typically a blend of soybean, rapeseed, or corn oils — used for everything from frying to dressings. The category was created in the 1920s by Japanese food companies marketing oil pure enough to eat raw. The で marks instrumental means: the oil is what the frying happens *in*. The verb 揚げる specifically means to deep-fry (as opposed to 焼く 'pan-fry/grill' or 炒める 'stir-fry'), so this sentence implies the fish was fully submerged in hot oil — think tempura or 唐揚げ.