$P = \sigma A$
where:
$P = 400 \, \text{kN} = 400\,000 \text{N}$
$\sigma = 120 \, \text{MPa}$
$A = \frac{1}{4} \pi D^2 - \frac{1}{4} \pi (100^2)$
$A = \frac{1}{4} \pi (D^2 - 10\,000)$
Thus,
$400\,000 = 120 \, [ \, \frac{1}{4} \pi (D^2 - 10\,000) \, ]$
$400\,000 = 30 \pi D^2 - 300\,000 \pi$
$D^2 = \dfrac{400\,000 + 300\,000 \pi}{30 \pi}$
$D = 119.35 \, \text{ mm}$ answer