The open-process-close discipline requires closing a file even when processing fails partway through. Why does that matter, when the program is ending anyway?

D. Because buffered writes may be lost and file handles are a limited resource.

Two distinct problems: writes sit in a buffer until it fills or the file closes, and handles are a finite per-process resource that a long-running program can exhaust. Data does go to disk progressively, so closing flushes what remains rather than writing everything. The failure path is easy to forget, which is why with, using, try-with-resources and RAII all exist.