To determine which option has the greatest magnitude of force, we need to consider the weight of each object, as the force required to lift an object is equal to the weight of that object. Generally speaking, in descending order of expected weight, we have:
a. the force needed to lift a textbook (usually the heaviest of the options) b. the force needed to lift a chair (can vary, but typically lighter than a textbook) c. the force needed to lift a hairbrush (usually lighter than a chair) d. the force needed to lift a pencil (the lightest)
Based on this reasoning, option a (the force needed to lift a textbook) would typically require the greatest magnitude of force.