Step 1: Understanding the structure.
The blank must refer to something that guides us to Truth. Among the given choices, rationality is the most fitting term.
Step 2: Checking word pairing.
(A) trusted guide: A trusted guide to Truth makes perfect contextual sense.
(B) appointed guide: Not meaningful in philosophical context.
(C) revered guide: Philosophy is revered but doesn’t fit “we allow ourselves to be swept by it.”
(D) favourite guide: Sounds informal and incorrect.
Step 3: Conclusion.
“Rationality is our trusted guide” is the only grammatically and logically correct combination.