The correct answer is D) garter springs. Garter springs are used to secure the carbon ring packing segments in the shaft gland assembly of a steam turbine. These springs apply constant pressure to the carbon rings, ensuring a tight seal around the turbine shaft and preventing steam leakage. The other options are incorrect because: A) Centering rings are used to align the packing segments, but do not secure them. B) Labyrinth rings create a tortuous path for steam leakage, but do not directly secure the packing segments. C) Steam pressure alone is not sufficient to maintain the necessary seal; the garter springs provide the required compressive force.