Which one of the following statements related to lanthanons is incorrect?

Answer: 2. All the lanthanons are much more reactive than aluminium.

Key idea: check each statement against NCERT facts on lanthanoids.

• Statement 1: \(Ln(OH)_3\) basicity falls from Pr to Lu as the ionic radius shrinks (lanthanoid contraction). Correct.
• Statement 3: \(Ce^{4+}\) is a strong oxidant, used in volumetric analysis. Correct.
• Statement 4: Eu shows +2 (stable \(f^7\)). Correct.
• Statement 2: the key treats "all lanthanoids are much more reactive than aluminium" as the incorrect one.

The statement is the keyed incorrect one because it claims all lanthanoids are much more reactive than Al, which is too sweeping (later members are less reactive); the other three statements are straightforwardly true.