The correct answer is “but”.
The phrase “The paths of glory lead but to the grave” is from Thomas Gray’s poem “Elegy Written in a Country Churchyard”. The poem is about the fleeting nature of life and the inevitability of death. The phrase “The paths of glory lead but to the grave” suggests that even the most glorious achievements in life ultimately lead to death.
The other options are not as relevant to the meaning of the poem. Option A, “straight”, suggests that the paths of glory are easy to follow. However, the poem suggests that the paths of glory are actually quite difficult to follow. Option B, “but”, suggests that there is something else that happens after death. However, the poem does not suggest that there is anything else that happens after death. Option C, “in”, suggests that the paths of glory are within reach. However, the poem suggests that the paths of glory are actually quite far away. Option D, “directly”, suggests that the paths of glory lead directly to the grave. However, the poem suggests that the paths of glory are actually quite winding.