Explanation:

• "Redoubtable" means inspiring fear or awe, commanding respect, or formidable in character.
• "Formidable" shares similar meanings with "redoubtable" as it also denotes something inspiring fear or respect due to its power, strength, or excellence.
• "Flimsy" means weak or easily broken, which is the opposite of  "redoubtable."
• "Perplexing" means confusing or puzzling, which is unrelated to the concept of being formidable or inspiring respect.
• "Voluble" means speaking fluently or readily, which is also unrelated to the concept of being redoubtable.

Therefore, the correct synonym for "redoubtable" is formidable.