The question targets the radiographic definition of bulky mediastinal disease used in Ann Arbor staging of Hodgkin lymphoma, flagged by the letter $X$.
Mechanically, the radiologist measures the greatest transverse width of the mediastinal mass and compares it to the maximum internal transverse diameter of the thorax on a standing posteroanterior chest film, conventionally at the $T5$-$T6$ level. When the ratio exceeds $\frac{1}{3}$, the mass qualifies as bulky and the stage gains the $X$ suffix. An alternative size-based criterion labels any single nodal mass of $\geq 10\,\text{cm}$ as bulky.
Bulk matters because it predicts higher relapse risk and frequently mandates the addition of radiotherapy to chemotherapy. The other listed fractions ($\frac{1}{2}$, $\frac{1}{4}$, $\frac{2}{3}$) are distractors with no basis in the staging rules.
\[\boxed{\text{Bulky mediastinal mass} = \text{width} > \tfrac{1}{3}\ \text{intrathoracic diameter}}\]