Explanation:

Both (a) and (b).

Section 18 is not applicable to Simple admission of a debt as according to Section 18 the suit or application must be in respect of any property or right and it must be written and signed by the property.
Section 18 lays down that Effect of acknowledgment in writing.—(1) Where, before the expiration of the prescribed period for a suit or application in respect of any property or right, an acknowledgment of liability in respect of such property or right has been made in writing signed by the party against whom such property or right is claimed, or by any person through whom he derives his title or liability, a fresh period of limitation shall be computed from the time when the acknowledgment was so signed.

Section 18 is also not applicable to execution of a decree or order as explanation 'c' of Section 18 lays down that:
(c) an application for the execution of a decree or order shall not be deemed to be an application in respect of any property or right.