Which structure is responsible for the formation of the sigmoid notch?

The trochlear notch of the ulna articulates with the trochlea of which bone?

Each human limb is made of (in $bones$)

The sigmoid notch is present in

$A$: Tibia is stronger and is present medially,whereas fibula is the outer and slender bone of the lower leg or shank.
$R$: Tibia has a sharp crest in the shaft and a projection on the inner side of the ankle called the lateral malleolus.

Which of the following bones form the pectoral girdle?