Most common organism causing T. capitis is –: (A) Trichophyton tonsurans, (B) Microsporum, (C) Epidermophyton, (D) Candida albicans
Answer is B. "Ringworm of the scalp occurs in children exclusively and is mainly due to M. canis".
Following statement of Harrison creats some confusion here :-
"The predominent organism is T tonsurans".

Therefore, I had to do extensive search on this question and I collected following information :
Most common organism causing Ectothrix is M. canis, whereas most common organism causing Endothrix is T tonsurans. Ectothrix is the most common type of T. capitis, therefore M. canis would be the most common cause of T. capitis.