A most malignant form of NHL is -
Diffuse large cell lymphoma is most malignant, amongst the options provided. Although Follicular large cell and Diffuse varieties are both classified in the intermediate grade of the 'working formulation of NHL' in general, "lymphomas with a follicular histological pattern are of a Lower grade (longer survival times) than those with a diffuse pattern".
Following information have been added in 8th  of Robbins

Diffuse large B-cell lymphoma is the most common form of Non-Hodgkin lymphoma.
Diffuse large cell
Small cell lymphocytic lymphoma
Mantle cell lymphoma
Large cell follicular