$\large\frac{\beta '}{\beta} =\large\frac{\Large\frac{\lambda D'}{d}}{\Large\frac{\lambda D}{d}}$

$\qquad= \large\frac{D'}{D}$

$D= 75$

$D'= 75+25=100$

$\large\frac{\beta'}{\beta}= \large\frac{100}{75}$

$\beta' =0.05 \times \large\frac{4}{3}\; $$mm$

$\qquad=0.066 \;mm$

Hence b is the correct answer.