Given below are two statements. One is labelled as Assertion (A) and the other is labelled as reason (R). 

Both (A) and (R) are true and (R) is the correct explanation of (A)

Outer electronic configuration of lanthanides : 4f^1-145d^0-26s²

Outer electronic configuration of actinides      : 5f^1-146d^0-27s²

size of 5f orbitals are more than size of 4f orbitals. Due to large size of 5f orbitals are more diffused  than 4f orbitals and shielding effect of 5f electrons are less than shielding effect of 4f electrons. Thus lanthanide contraction is weaker than actinide contraction.