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Chocolate has the ability to prevent LDL oxidation.
Literal
Chocolate [in-には] LDL [genitive-の] oxidation [object-を] prevent power [subject-が] exists.
A health-science factoid. ~力がある (literally 'has the power to X') is a set predicate pattern meaning 'is capable of X,' 'has the ability to X.' The power here is marked by が (the subject of the state-verb ある), and the thing it acts on (oxidation) is the object of the verb 防止 ('prevent'). 防止する → 防止 as a noun/verbal-noun, here used as a pre-modifier: LDLの酸化を防止する力 = 'the power to prevent LDL oxidation.' A classic dense modifier chain. LDL is the loanword abbreviation for low-density lipoprotein (bad cholesterol).