A reduction in atomic size with increase in atomic number is a characterstic of elements of

$\begin{array}{1 1}(a)\;d-block&(b)\;f-block\\(c)\;radio\;active\;series&(d)\;high\;atomic\;masses\end{array}$

A reduction in atomic size with increase in atomic number is a characterstic of elements of f-block.
Hence (b) is the correct answer.