arc / ark are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
arc: (noun) a continuous portion of a circle.
ark: (noun) (Judaism) sacred chest where the ancient Hebrews kept the two tablets containing the Ten Commandments. (noun) a boat built by Noah to save his family and animals from the flood.
arc: AA·r·k
ark: AA·r·k
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)