Which of the following relational algebraic expressions is equivalent to the SQL query SELECT X FROM B WHERE Y ?

Explanation

The SQL query filters rows of B that satisfy condition Y (a selection on Y) and then returns only column X from the surviving rows (a projection onto X). In relational algebra this is written as the projection applied to the selection, i.e. π_x(σ_y(B)), since the WHERE clause is always applied before the SELECT list restricts the output columns. The other options either apply projection before selection, or apply both operators on the same attribute, which does not correspond to this query.