Answer:
3. Correctly ignored a sunk cost
Explanation:
Sunk costs refer to those costs which have been incurred in the past, which are non recoverable and which have no current or future benefits.
Sunk costs are considered as irrelevant for decision making process as they do not relate to current period and have no future implications. For example, research and development expenditure incurred in the past represents a sunk cost.
In the given case, the ticket for opera was already purchased for $100 which can now neither be recovered nor transferred. Thus this cost is irrelevant for decision making as expenditure has already been made. When Shen decided to go for a party instead of the concert, Shen has correctly ignored a sunk cost.
