What is the most likely diagnosis?

A 19-year-old man was seen in the diabetes clinic. He had lost 2 kg in weight since the diagnosis of diabetes mellitus 18 months previously. At presentation, his body mass index was 33 kg/m2 (18C25), his random plasma glucose was 18.0 mmol/L and his haemoglobin A1c was 56 mmol/mol (20C42). He was taking gliclazide, and metformin had been added later. His father and grandfather had developed diabetes mellitus during their twenties.

Investigations:

haemoglobin A1c56 mmol/mol (20C42)

serum C-peptide301 pmol/L (180C360)

anti-glutamic acid decarboxylase (GAD)

antibodiesnegative

What is the most likely diagnosis?
A . chronic pancreatitis
B . latent-onset diabetes of autoimmunity
C . maturity-onset diabetes of the young
D . type 1 diabetes mellitus
E . type 2 diabetes mellitus