1. Problem statement: Complete the sentence using one word from the box: apply, application, deal with, attach, look forward to. 2. Formula and rules: Use the sentence structure Subject + Verb + Object. 3. Key rule: Choose a word that is a verb when a verb is needed and that naturally collocates with "copies" and "diplomas". 4. Check options one by one: 4.1 "apply" is a verb meaning to request and needs an object like "for a job", so "I apply copies" is ungrammatical. 4.2 "application" is a noun, so it cannot directly fill a verb slot after the subject in this sentence. 4.3 "deal with" is a phrasal verb meaning to handle, but "I deal with copies" is possible yet less natural in the context of submitting documents. 4.4 "attach" is a verb meaning to include with, and it collocates naturally with "copies", making "I attach copies of my diplomas" correct and natural. 4.5 "look forward to" is a phrasal verb that requires a noun or gerund after it, so it does not fit here. 5. Final answer: The best choice is "attach". 6. Completed sentence: "I attach copies of my diplomas."