An I-beam cross section is shaped with wide flanges at the top and bottom connected by a thin web. This shape is efficient because

Correct answer to question 191 (Mechanical Comprehension): most of the bending resistance comes from material far from the centre, where the flanges are

Method

Bending resistance depends heavily on material placed far from the neutral axis, so putting extra material in the flanges resists bending efficiently while the web mainly handles shear.