On physical interceptions, with SM-3 missiles. The primary purpose of the SM-3 system is to defend against ballistic missile threats in the mid-course phase, which is the time when the missile is traveling outside the Earth's atmosphere, heading towards Earth. DF-21Ds are medium range ASBMs and well within SM-3 interception range and altitude. Besides, the DF-21Ds are simple MRBMs that are copied from the US Pershing II MRBM phased-out in 1989.

The main difference lies in the targeting radar between the missiles: Pershing uses radar-based DSMAC system while the DF-21D uses a regular radar acquisiton and tracking system as similarly used in Harpoon anti-ship missiles.

The SM-3 (Standard Missile-3) is a ship-based ballistic missile defense system developed by the United States. It is designed to provide mid-course interception capability against short- to intermediate-range ballistic missiles. The SM-3 is part of the Aegis Ballistic Missile Defense System, which integrates various sensors, command and control systems, and interceptors to detect, track, and destroy incoming ballistic missiles.

It must be noted that DF-21D launches in China are immediately detected by geo-stationary US satellites that are always pointing towards China. This gives ample warning time for the AEGIS system deployed on the Carrier Battle Group to begin tracking the incoming missiles.

US satelite eyes on China

AEGIS system tracking missile launches from rogue countries

Other than physical interception, US Navy Carrier Battle Group will attempt to jam the DF-21D radar at the terminal phase in the last 30s, should the physical interception fail. SM-6s will most likely also be launched at terminal phase for point defense, if SM-3s fail to intercept at the mid-course phase.