Answer
See explanation below.
Work Step by Step
Given that the half-life of plutonium-239 is 2.41 x 10^4 yr.
And given that seven half-lives are required to reach a tolerable level of radioactivity.
So time 239 Pu can be stored is = 7 x half life
= 7 x (2.41 x 10^4 yr) = 1.69 x 10^5 yr