This guide provides the solution and grammatical analysis for from the 3rd-grade Russian language textbook by T. G. Ramzaeva (specifically the edition from the early 2000s). Exercise Text: "Олени в заповедниках"
Case: — Caring about whom? About deer (Object with preposition "об"). Они подкармливают оленей зимой. This guide provides the solution and grammatical analysis
Case: — Feeding whom? Deer (Direct object). 2. Grammar Task: Noun Declension This guide provides the solution and grammatical analysis
(Nature Reserve): A protected area where animals and plants are kept safe from hunting and industrial activity. This guide provides the solution and grammatical analysis
The word "олени" in the first sentence is the subject, so it must be in the Nominative case.