Which one of the following can be considered as an initial warning of an approaching Tsunami ?

The most distinct natural warning sign of an approaching tsunami is the rapid withdrawal of water away from the beach, often referred to as 'drawback'. This phenomenon occurs when the trough of the tsunami wave reaches the shore before the crest [c1, c2]. This recession can expose hundreds of meters of the seabed that are normally submerged, sometimes stranding marine life [c2, t2]. While a roaring or thundering sound may also precede a tsunami's arrival [t3], the dramatic recession of the sea is a primary visual indicator that provides a critical, albeit short, window for evacuation [t5]. Squally winds and rainfall are associated with meteorological events like thunderstorms or 'meteotsunamis' rather than seismic tsunamis [t4, t7]. Rapid landward movement represents the actual inundation phase rather than an initial warning sign [t5].

Sources

1. [1] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 17: Contemporary Issues > Signs of an Approaching Tsunami > p. 16
2. [2] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 17: Contemporary Issues > Warnings and Prevention > p. 17
3. [3] https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00031-what-are-thunderstorms.html