The growing point of the new sporophyte in the shown fern prothallus is labelled as:

Explanation

In the life cycle of ferns, the prothallus is the gametophyte stage, which is small and heart-shaped. The new sporophyte begins to grow from the fertilized egg located on the prothallus. The 'growing point' is where this new sporophyte develops, typically labeled as 'A' in diagrams.