Answer to b
i. Chart b contains two diagonal lines that cross at the central black point. In patient who cannot see the black spot because of central scotoma, the lines help to maintain fixation and allows the patient to outline the limits of the scotoma.
ii. The red lines on a black background in chart c is useful for patient with 
neuro-ophthalmological disorders such as optic nerve or chiasmal lesion or toxic amblyopia.
 

Answer to c
i. A paracentral scotoma. This can occur with chorioretinal scars such as in 
toxoplasmosis or congenital disorders such as congenital hypertrophy of retinal pigment epithelium.
ii. An arcuate scotoma. Glaucoma patient.
iii. Central distortion with micropsia (the squares look smaller). This occurs in conditions that force the cones apart so that the retinal images will stimulate fewer cones than normal. Consequently, the lines and the squares of the Amsler's chart are seen as smaller and tend to bend away towards the centre. Example: diabetic cystoid macular oedema.