The connecting rod joins the piston to the crankshaft and, on every power stroke, carries a large compressive push while also resisting the bending and buckling forces set up as it swings.

The shape of its shank must therefore give maximum resistance to buckling for the least possible weight, because the rod is a fast-moving part whose mass creates inertia loads.

An I-section does this best: it concentrates most of the material in two flanges placed far from the central neutral axis, which is where material contributes most to stiffness and buckling resistance.

Keeping the middle thin removes weight that would add little strength, so the rod stays light yet remains rigid in the plane where buckling is most likely.

A circular section (option 3) or a square section (option 2) puts a lot of metal near the axis where it does little to resist bending, making the rod needlessly heavy for the same strength.

A T-section (option 1) is unsymmetrical and would carry load unequally about the axis, so it is unsuitable for a part that is pushed hard in both directions.

Hence, the answer is I-section.