How is a generator expression defined in Python?

The correct answer is Option B: Using parentheses (...).
A generator expression in Python is defined using parentheses (...) syntax. Inside the parentheses, you write an expression that generates values, similar to a list comprehension but with parentheses instead of square brackets. The expression can be a simple expression or a more complex one involving conditionals or nested expressions. When iterated over, a generator expression produces values lazily, one at a time, without storing the entire sequence in memory at once. This lazy evaluation mechanism makes generator expressions efficient for handling large datasets or infinite sequences.