(C) Sodium
The corrosive warning sign is displayed on containers of sodium hydroxide (NaOH) pellets, as it is a strongly corrosive alkali.
Source: Chapter 2, Section 2.2.1 (Fig. 2.5)
The textbook explicitly mentions the corrosive warning sign (Fig. 2.5) in the context of concentrated sulphuric acid and sodium hydroxide pellets. Among the four options, only sodium hydroxide (NaOH) is referenced with this warning sign. Ca(OH)₂, Mg(OH)₂, and Al(OH)₃ are either insoluble, weak, or amphoteric — not strongly corrosive alkalis that require this label.