Dictionary with Single Key and Multiple Values Example in C#. See https://repl.it/CY3S/1 to run example code online.

multiple_value_dictionary.cs

using System;
using System.Collections.Generic;

class MainClass {
  public static void Main (string[] args) {
    
    Dictionary<string, List<String>> map = new Dictionary<string, List<String>>();
    
    // create list one and store values
    List<string> valSetOne = new List<string>();
    valSetOne.Add("Apple");
    valSetOne.Add("Aeroplane");
    
    // create list two and store values
    List<String> valSetTwo = new List<String>();
    valSetTwo.Add("Bat");
    valSetTwo.Add("Banana");
    
    // create list three and store values
    List<String> valSetThree = new List<String>();
    valSetThree.Add("Cat");
    valSetThree.Add("Car");
    
    // add values into map
    map.Add("A", valSetOne);
    map.Add("B", valSetTwo);
    map.Add("C", valSetThree);
  
    // iterate and display values
    foreach(KeyValuePair<string, List<string>> kvp in map){
      foreach(string value in kvp.Value){
         Console.WriteLine("Key = {0}, Value = {1}", kvp.Key, value);
      }
    }
  }
}

How can I delete 'Aeroplane' from valSetOne after adding into map?

Here's how you can delete value from multi-valued dictionary,

dictionaryName["KeyThatContainsThatValue"].RemoveAt(indexOftheValue);

Example :
map["A"].RemoveAt(1);

And how can I update only 1 value in a multi value dictionary?

Now I want to add 4th Item which is a single string (not a list) in the same (map) dictionary. How can I do this?

Using new MultiValueDictionary is an option: https://www.nuget.org/packages/Microsoft.Experimental.Collections

Out of the whole Dictionary I only want Aeroplane. How do I do that?