SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/retrieve-related-item-with-single-related-item-limitation

RetrieveRelatedDataItems.cs

using System.Linq;
using Telerik.Sitefinity.Model;
using Telerik.Sitefinity.News.Model;

namespace SitefinityWebApp
{
    public class RetrieveRelatedDataItems
    {
        /// <summary>
        /// Retrieve a single item, using the RelatedDataPropertyDescriptor in the following way
        /// </summary>
        /// <param name="item">The parent item</param>
        /// <returns>The related speaker</returns>
        public IDataItem GetRelatedSpeaker(IDynamicFieldsContainer item)
        {
            IDataItem relatedSpeaker = item.GetValue<IDataItem>("Speaker");

            return relatedSpeaker;
        }
    }
}