After labelling the figure, we get the following,

• Simplest triangles EFH, BIC, GHJ, GIJ, EKD and CKD (6)
• Triangles with two components each ABJ, AFJ, GCK, GEK, CED and GHI (6)
• Triangles with three components each GCD, GED, DJB and DJF (4)
• Triangles with four components each ABF and GCE (2)
• Triangles with five components each ABD and AFD (2)
• Triangles with six components is FBD (1)

Total number of triangles in the figure is 6 + 6 + 4 + 2 + 2 + 1 = 21

Hence option D is correct.