A subquery is called a single-row subquery when
A.
the inner query returns a single value to the main query
B.
the inner query uses an aggregate function and returns one or more values
C.
there is only one inner query in the main query and the inner query returns one or more values
D.
the inner query returns one or more values and the main query returns a single value as output
Single-row VS multiple-row subqueries
Subqueries that can return only one or zero rows to the outer statement are called single-row subqueries. Single-row subqueries are subqueries used with a comparison operator in a WHERE, or HAVING clause.
Subqueries that can return more than one row (but only one column) to the outer statement are called multiple-row subqueries. Multiple-row subqueries are subqueries used with an IN, ANY, or ALL clause.