What is the effect of lanthanoid contraction in the lanthanoid series of elements with increasing atomic number?

Explain the lanthanoid contraction.

Which of the following compounds is the most acidic?

The main reason for the actinoids exhibiting a larger number of oxidation states than the lanthanoids is:

Which statement is incorrect with reference to inner transition elements?

Given below are two statements:
Statement $I$: In the Lanthanoids,the formation of $Ce^{+4}$ is favoured by its noble gas configuration.
Statement $II$: $Ce^{+4}$ is a strong oxidant reverting to the common $+3$ state.
In the light of the above statements,choose the most appropriate answer from the options given below: