Final answer:
Polydipsia, polyuria, and polyphagia in combination are indicative of diabetes mellitus, which is the right answer in the given multiple choice question.
Step-by-step explanation:
Polydipsia, polyuria, and polyphagia, when presenting simultaneously in an athlete, are classic signs and symptoms of diabetes mellitus. Polyuria is a condition characterized by frequent urination, which leads to dehydration; polydipsia is an increased intake of water due to the dehydration caused by polyuria; and polyphagia is an increased intake of food due to the cells being deprived of glucose and essentially 'starving', despite high blood sugar levels. These symptoms occur because the kidneys are forced to filter and excrete an excessive amount of glucose, drawing water out with it and consequently leading to increased urine production and thirst, while the lack of glucose uptake by cells triggers excessive hunger.
Therefore, the correct answer to the question is 'd) Diabetes mellitus', as the simultaneous presence of these three symptoms strongly suggests this condition, especially in the context of persistently high blood glucose levels.