Identify the open-ended question from among the following :

In teaching, questions are classified as open-ended or closed-ended based on how many valid answers they admit. A closed-ended question settles into exactly one correct, verifiable fact that a learner recalls or looks up. An open-ended question, in contrast, has no single fixed answer — it asks the learner to reason, predict, or speculate, so several different, justified responses can all be valid.

Among the four questions, "What would happen if humans could digest cellulose?" is a hypothetical "what would happen if...?" question. It does not point to one recorded fact; instead it invites a learner to reason out possible consequences — for example, being able to draw nutrition directly from plant fibre, or a change in dietary needs and food choices. Because it opens up reasoning toward multiple justified outcomes rather than settling on one fact, it is the open-ended question.

• Why can humans not digest cellulose? — settles into one fixed biological fact: human digestive enzymes cannot break the beta-1,4 glycosidic bonds linking glucose units in cellulose, so it is closed-ended.

• What is cellulose made of? — asks for one definite compositional fact: cellulose is a long-chain polysaccharide of glucose units, so it is closed-ended.

• Name any five organisms that can digest cellulose. — even though several organisms qualify, each valid answer (e.g. cow, goat, buffalo, termite, certain gut microbes) is still one exact fact to recall rather than a reasoned prediction, so it remains closed-ended.