Which ingredient provides the signature crunch in a Taco Bell taco?

The ingredient that provides the signature crunch in a Taco Bell taco is crispy tortillas. When tacos are assembled, they use either soft tortillas or crispy corn tortillas that are fried to create that distinct crunch. This texture is essential in giving the taco a satisfying bite and contributes to the overall experience of enjoying the product.

Shredded lettuce, while it may add a refreshing crunch and layer of flavor, does not provide the foundational crunch that comes from the tortilla itself. Chopped onions add flavor but do not contribute to the crunch. Refried beans offer a creamy and soft texture, which contrasts with the desired crispy element in a taco. Therefore, the characteristic crunch that defines a Taco Bell taco primarily comes from the crispy tortilla, making it the correct choice.