1. Add the abstract keyword before the Animal class definition to make it abstract.
2. Add the abstract keyword before the makeSound method's return type in the Animal class to make the method abstract. Make sure to remove the method's body from the Animal class leaving only it's blueprint (return type, name and arguments) : returnType methodName(arg1, arg2, ..);

    using System;

    abstract class Animal
    {
        protected string species;
    
        public Animal(string species)
        {
            this.species = species;
        }
    
        public abstract void MakeSound();
    
        public void DisplaySpecies()
        {
            Console.WriteLine($"Species: {species}");
        }
    }
    
    class Cat : Animal
    {
        string furPattern;
    
        public Cat(string species, string furPattern) : base(species)
        {
            this.furPattern = furPattern;
        }
    
        public override void MakeSound()
        {
            Console.WriteLine("Meow! Meow!");
        }
    }
    
    class Dog : Animal
    {
        float weight;
    
        public Dog(string species, float weight) : base(species)
        {
            this.weight = weight;
        }
    
        public override void MakeSound()
        {
            Console.WriteLine("Woof! Woof!");
        }
    }
    
    class ConsoleApp
    {
        static void Main()
        {
            Animal myCat = new Cat("Feline", "Ginger & White");
            Animal myDog = new Dog("Canine", 42.5f);
    
            myCat.DisplaySpecies();
            myCat.MakeSound();
    
            Console.WriteLine("\n");
    
            myDog.DisplaySpecies();
            myDog.MakeSound();
        }
    }