Not a disorder of form of thought is:

C i.e. Thought Block Formal thought disorder typically refer to marked abnormalities in the form & flow of or connectiity of thought; some clinicians use the term broadly to include any psychotic cognitive sign or symptoms. So broadly speaking all can be included in formal thought disorder but in tangentiality, loosening of association and derailment the form of thought is more disordered than thought blockQ. (Kaplan's & CDTP) In formal thought disorder, the structure or form of thought is disordered. It includes perseveration, flight of ideas (clang association, puns, rhymes), loosening of associations, derailmentQ, vorbeireden, verbigeration (word salad) (Oxford); tangentialityQ, transitory thinking, substitution, omission, drivelling, desultory thinking, neologism, povey of speech and povey of content (Fish). Whereas thought block is mainly a disorder of flow/stream of thoughtQ.