One of the structures shown has degree of unsaturation 4 and is acyclic. Which one?

Acyclic with DBE 4 needs four pi bonds worth. Structure A (hexa-2,4-diyne, $\mathrm{C}{}_6\mathrm{H}{}_6$) has two triple bonds, DBE 4, no ring. Structure B (benzene) has DBE 4 but a ring. C (pent-1-en-4-yne) has DBE 3 and D (cyclohexa-1,4-diene) has DBE 3. The reference drawn under the question is hexa-1,3,5-triene (DBE 3) and is not an option.