The incorrect quotation is 'alike', because in the sentence "Cotton fiber, 'alike' 'other' vegetable fibers, 'is composed' 'mostly' of cellulose," the word "alike" should not be quoted and is used incorrectly; a more suitable word would be "like" or "similar to". 

Here's the breakdown: 

• 'alike': This word is incorrectly quoted. The word "alike" is an adjective, and in this context, it's likely meant to be a comparative adverb or preposition. The correct phrasing would be "like other" or "similar to other" vegetable fibers.
• 'other': This word is not incorrect but is unnecessary to quote.
• 'is composed': This is not incorrect and does not need to be quoted.
• 'mostly': This word is not incorrect and does not need to be quoted.