C# - Making a class thread safe

Question

I have been given the following code and tasked with making it thread safe:

public class HashtableWrapper
{

    private Hashtable _cachedData;

    public HashtableWrapper()
    {
        _cachedData = new Hashtable();
    }

    public void Add(object key, object value)
    {
        _cachedData[key] = value;
    }

    public object Get(object key)
    {
        return _cachedData[key];
    }
}

I am not that well versed in threading but the following is some 'solutions' I have found/come up with:

• Would it have something to do with making the Add and Get methods synchronised?

• Making _cachedData static?

• Creating a synchronised wrapper around Hashtable: https://learn.microsoft.com/en-us/dotnet/api/system.collections.hashtable.synchronized?view=netcore-3.1 (Unsure where to do this if this was the solution - after _cachedData = new Hashtable(); maybe?)

• Using a lock object, like here: Making class fields thread-safe - for example:

  private static object lockObject = new Object();
  
  public void Add(object key, object value)
    {
        lock(lockObject)
        {
            _cachedData[key] = value;
        }
    }