He started asking me personal questions, so I told him, "Mind your own business".
The two ways of relating what a person has said are 1)direct and 2)indirect (reported speech).In direct speech, the original speaker's exact words are repeated. Direct speech is found in plays and quotations. In indirect/reported speech, the exact meaning is expressed but not necessarily using the original speaker's exact words. Pronouns and possessive adjectives usually change from first or second person to the third person except when the speaker is reporting his own words. Sometimes a noun can be inserted to avoid doubt.
Option C is correct as the given statement is a command and to transform a command to indirect speech, the introductory verb 'order' is used with 'to' , and 'your own' is changed to 'his own'.
Options A, B, and D are wrong as 'told', 'advised', and 'that cannot be used in the transformation.
The correct answer is C)I ordered him to mind his own business.