= Solution
For high-strength <steel>, use
$$
\boxed{t_{\max,\rm high}=\frac1\pi
\left(\frac{K_{IC,\rm high}}{Y\sigma_{y,\rm high}}\right)^2,
\qquad t_{\rm yield,\rm high}\simeq\frac{7\,\mathrm{MPa\,m}}{\sigma_{y,\rm high}}.}
$$
At fixed toughness, raising yield <stress> reduces $t_{\max}$ quadratically but reduces the required pressure thickness only linearly. Thus a higher-strength grade can have a worse leak-before-break screening margin. The relevant screening combination is $K_{IC}^2/\sigma_y$, since simultaneous satisfaction requires
$$
\boxed{\frac{K_{IC}^2}{\sigma_y}>\pi Y^2Pr
\simeq22.0Y^2\,\mathrm{MPa\,m}.}
$$
Use measured low-temperature toughness and appropriate joining/environmental performance; strength alone does not establish suitability. The comparison also has to remain within the thin-wall approximation.
\b[TODO: numerical thickness cannot be completed without the unavailable 2018 NST Data Book's high-strength-steel property pair.] The four material rankings cannot be made numerically definitive from the exam PDF alone.
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