Radius (left) and ulna (middle)

Question 1

36 and 39

Answer 1

Radial notch (36)  articulates with the ulna to form the proximal radioulnar joint (39)

Question 2

37 and 40

Answer 2

Ulnar notch of the radius (37)  articulates with the ulnar to form the distal radioulnar joint (40)

Question 3

38

Answer 3

Olecranon process  protrusion on the posterior surface

Question 4

41

Answer 4

Troclea notch  articulates with the trochlea of the humerus