The correct answer is B) When a subquery returns multiple values but the outer query expects a single value.

The “Subquery returned more than 1 value” error is a common error in SQL that occurs when a subquery returns multiple values but the outer query is designed to handle only a single value. This can happen when using the equal (=) operator with a subquery or when using subqueries in other comparison operators like IN or EXISTS.

For example, consider the following query:

SELECT name FROM customers WHERE customer_id = (SELECT customer_id FROM orders WHERE order_total > 1000)

If the subquery returns multiple customer_id values, the error will be thrown because the equal (=) operator expects only one value. To fix this error, you can use a different comparison operator or change the subquery to return a single value using the TOP or MAX function.