Which of the following factors may be regarded as the main cause of lanthanide contraction?

Which of the following factors may be regarded as the main cause of lanthanide contraction?
Which of the following factors may be regarded as the main cause of lanthanide contraction?
Which of the following factors may be regarded as the main cause of lanthanide contraction?
Which of the following factors may be regarded as the main cause of lanthanide contraction?

contraction?