An economy has to choose one of the many possibilities that it has at given resources. Therefore, the cost that an additional unit of X commodity in terms of the amount of the Y commodity that has to be forgone is called ................ cost.

The correct term to fill in the blank is opportunity cost.

When an economy operates with fixed resources and technology, it faces a fundamental trade-off: to increase the production of one commodity (X), it must divert resources away from the production of another commodity (Y). The amount of Y that must be sacrificed to produce one additional unit of X is the definition of opportunity cost.

Regarding "Economic Cost":

While you noted that "economic cost" is sometimes used interchangeably in casual conversation, in formal economic theory and examination contexts

• Opportunity Cost: Focuses on the trade-off and the value of the foregone alternative. It is the conceptually correct answer for the scenario described, as it directly addresses the "amount that has to be forgone."
• Economic Cost: Typically refers to the sum of explicit costs (actual out-of-pocket expenses) and implicit costs (the opportunity costs of using resources the firm already owns). Therefore, "opportunity cost" is the more precise and specific term for the trade-off mechanism described in your prompt.

Given the specific phrasing regarding the "amount of the Y commodity that has to be forgone," opportunity cost is the standard academic and universally accepted answer.