Final answer:
The correct sentence that substitutes the direct object with the correct pronoun is 'Me lo compro'.
Step-by-step explanation:
The correct sentence that substitutes the direct object with the correct pronoun is Me lo compro.
In this sentence, 'me' is the pronoun that represents the direct object 'el ratón del ordenador' (the computer mouse), and 'lo' is the pronoun that replaces the masculine noun 'ratón'.
For example, instead of saying 'Me compro el ratón del ordenador' (I buy the computer mouse), we can say 'Me lo compro' to replace the noun with the pronoun and maintain the same meaning of the sentence.
Learn more about Direct object pronouns in Spanish