Final answer:
Oliguria is the term used to describe the condition of a dog not urinating as often, which indicates an abnormally low amount of urine excretion, possibly due to various health issues. The correct option is D.
Step-by-step explanation:
The condition where a dog has not been urinating as often as it had previously is called oliguria.
Oliguria is defined as the excretion of less than 600 ml of urine per day. It can be caused by several conditions such as fever, shock, vomiting, and diarrhoea.
In contrast, anuria is the condition where there is no urine excretion or it is below 10 ml/day and is observed in conditions like acute renal failure and nephritis.
On the other hand, polyuria is the excretion of more than 2500 ml of urine per day and can be a sign of conditions such as diabetes insipidus or diabetes mellitus. The correct option is D.