Final answer:
The correct word to complete the sentence 'Mike has got new CDs and old ...' is '-ones'. This avoids repeating the noun 'CDs' and correctly agrees with the plural form.
Step-by-step explanation:
The correct answer to the question is -ones. In the sentence 'Mike has got new CDs and old -ones,' the word 'ones' is used to replace the noun 'CDs' to avoid repetition. The word 'ones' refers back to the CDs previously mentioned, thus 'ones' is the plural form of 'one.' The use of 'ones' here indicates that we are talking about more than one old CD, aligning with the plural noun 'CDs.'