Carat – a measure of weight used mainly for gemstones, equal to 200 mg.
Caret – a typographic mark shaped like an inverted V.
Carrot – a plant of the parsley family, or its edible root.
Karat – a measure of the percentage of gold in an alloy, equal to 4.1667%. (Though it is worth noting that outside the US, this spelling is not used. Instead, this AND the gemstone usage are both spelled "carat.")