Let the three angles be $3x,4x,5x$ degrees then
$3x+4x+5x=180^{\large\circ}$
$12x=180^{\large\circ}$
$x=\large\frac{180^{\large\circ}}{12}$
$x=15^{\large\circ}$
Smallest angle =$3x$
$\qquad\qquad\quad\;=3\times 15^{\large\circ}$
$\qquad\qquad\quad\;=45^{\large\circ}$
Greatest angle =$5x$
$\qquad\qquad\quad=5\times 15^{\large\circ}$
$\qquad\qquad\quad=75^{\large\circ}$
In radian $\Rightarrow 75^{\large\circ}\times\large\frac{\pi}{180^{\large\circ}}$
$\Rightarrow \large\frac{5\pi}{12}$ radians.
Hence (b) is the correct answer.