Select the option that best completes the analogy: Frugal : Extravagant :: Transparent : ?

The relation is a word : its opposite. Frugal and extravagant are opposites in spending, so transparent must be matched with its opposite, opaque — that which no light passes through. 'Clear' and 'Visible' are the traps, since both are near in meaning to transparent and therefore point the wrong way; 'Fragile' describes a different property of glass altogether.