a) Tablet containing 2.5 mg diphenoxylate
Diphenoxylate is found combined with atropine (UK: co-phenotrope, brand name Lomotil, which is a POM) in quantities of 2.5 mg diphenoxylate and 0.025 mg atropine.
Diphenoxylate is a synthetic derivative of pethidine with little to no analgesic activity. It is used to alleviate diarrhoea and for colostomy/ileostomy patients. It is combined with sub-clinical amounts of atropine to discourage abuse. (Although atropine itself is anti-muscarinic, 0.025mg per dose is sub-therapeutic.)
In the DDO there is a entry specific for co-phenotrope under Part II of the First Schedule (Preparations to which Ordinance applies with Modifications):
Section 16: "A preparation of diphenoxylate containing, per dosage unit, not more than 2.5 milligrams of diphenoxylate calculated as base and not less than 25 micrograms of atropine sulphate."
Because under Part III by the statement (s.22) "Any other preparation stated in Part II", it is by default also PIII DD. However the exam question only asks for P2DD to be written.
For the answer you would write down P1 and P2DD
Reference: Martindale and BNF