The correct answer is D. A routine or plan
Step-by-step explanation:
In the sentence "This new diet regime I am on is very hard to stick with because it is so strict" the word "regime" which is the unknown word, in this case, is defined by the context clues especially elements such as "new diet" or "very hard" that suggests the word "regime" refers to a plan, routine concerning diet or exercise the individual finds really hard to follow. Considering this, based on the context clues the word regime means "at routine or plan", rather than a government or ruler as this is not related to politics or one skill because it is not related to any ability but to a new routine or plan.