That one is difficult because it is very American and just looks wrong to me.
Defence and defense are different spellings of the same word. Defense is preferred in American English, and defence is preferred in all other main varieties of English, including Australian, British, and Canadian English. The spelling distinction extends to most derivatives of defence/defense, including defences/defenses and defenceless/defenseless. But the words defensive, defensiveness, and defensively have an s everywhere.
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