The section modulus Z of a rectangular section of width b and depth d is:

Why is this the answer?

For rectangle: I = bd³/12, ymax = d/2. Section modulus Z = I/ymax = (bd³/12)/(d/2) = bd²/6. Z represents the resistance to bending; higher Z means lower stress for same moment. Z = M/σmax from the flexure formula.

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