Final answer:
ATP is one thing that ONLY occurs in the light dependent reactions (LDR) of photosynthesis.
Step-by-step explanation:
One thing that ONLY occurs in the light dependent reactions (LDR) is the production of ATP (Adenosine triphosphate). ATP is a molecule that stores and releases energy, and it is generated during the light-dependent reactions of photosynthesis. Light energy is used to convert ADP (Adenosine diphosphate) and inorganic phosphate into ATP through a process called photophosphorylation.