Check out this article and this simple example. Quick translation of same to your classes …
var d1 = typeof(Task<>);
Type[] typeArgs = { typeof(Item) };
var makeme = d1.MakeGenericType(typeArgs);
object o = Activator.CreateInstance(makeme);
Per your edit: For that case, you can do this …
var d1 = Type.GetType("GenericTest.TaskA`1"); // GenericTest was my namespace, add yours
Type[] typeArgs = { typeof(Item) };
var makeme = d1.MakeGenericType(typeArgs);
object o = Activator.CreateInstance(makeme);
To see where I came up with backtick1 for the name of the generic class, see this article.
Note: if your generic class accepts multiple types, you must include the commas when you omit the type names, for example:
Type type = typeof(IReadOnlyDictionary<,>);