Microorganisms commonly causing diarrhoeal diseases are 

$\text{Correct answer: C}$ 

$\text{Option (A):}$ 

$\bullet$ $Bacillus$ $pertussis$ causes the bacterial disease pertussis in patients and not any diarrhoeal disease.

$\text{Option (B):}$ 

$\bullet$ $Clostridium$ $tetani$ is a soil bacteria that is the agent responsible for causing tetanus.

$\text{Option (C):}$ 

$\bullet$ All the pathogens including $Escherichia$ $coli$, $Giardia$, $Shigella$, $Campylobacter$ and $Salmonella$ are causative agents of diarrhoeal diseases.

$\text{Option (D):}$ 

$\bullet$ $Neisseria$ and $Treponema$ are sexually transmitted diseases and they do not cause diarrhoea.

Hence, the correct answer is option (C).