Explanation

The key clue is:

"could not rely on the accuracy of signal flags..."

The sentence is about ensuring that each watch officer accurately and carefully reported observations.

D. fidelity means faithfulness or exactness in reproducing or reporting something. "Fidelity" to what was actually seen is exactly what's required.

F. rigor means thoroughness, precision, and strict attention to detail. Reporting with rigor would make the observations dependable.

A. discretion - careful judgment or tact, not accuracy.
B. steadfastness - loyalty or firmness, not precise reporting.
C. distinction - difference or excellence, unrelated here.
E. persistence - continuing despite difficulty, not about accurate observation.

Answer: D and F.