The term 'CAST ONE'S VOTE' is a phrase used to describe the act of voting in an election. It is composed of two words, 'OPT' and 'CHOOSE', which are both 3 and 6 letters long respectively. 'OPT' is defined as to select or make a choice from a range of possibilities, while 'CHOOSE' is defined as to pick out or decide on from a range of alternatives. Therefore, when combined, the phrase 'CAST ONE'S VOTE' can be interpreted as making a decision to select a candidate or party in an election.