Concept:The word 'Millennium' refers to a period of one thousand years. It is commonly misspelled because it has double 'l' and double 'n'.Explanation:Option A 'Milenium' has only one 'l' – wrong.Option C 'Millenniam' ends with 'iam' instead of 'ium' – wrong.Option D 'Millenium' has only one 'n' – wrong.Only option B 'Millennium' has both double 'l' and double 'n'.Thus, the correct spelling is 'Millennium'.Answer:The correctly spelt word is Millennium (Option B).