* "Mines" is the possessive form of the pronoun "mine", and it's used to indicate something belonging to the speaker.
* However, "mines" is usually used for plural possessions, like "Those are my mines" (referring to multiple mines).
To correct the statement, you need to use the singular possessive pronoun "mine" instead:
"That is mine."
This statement correctly indicates that the object in question belongs to the speaker.