Correct option is C
A: “Neither the rate nor the fees was discussed…” → With “neither…nor,” the verb agrees with the nearer subject. “fees” is plural, so use “were discussed.” Hence A–E.
B: “There is few evidence…” → “evidence” is uncountable, so use “little,” not “few.” “There is little evidence…” is correct. Hence B–D.
C: “Unless the fields are complete, the system will not proceed.” → Correct as written. Replacing with F (“are not complete”) creates a double negative with “unless” and flips the meaning, so C–F is wrong.