Secretion is the function of 

Parenchyma cells are involved in various functions, including storage, photosynthesis, and secretion. Some parenchyma cells can indeed secrete substances, but this is not their primary function.

Apical meristems are responsible for the primary growth of plants, producing new cells for the plant's lengthening. They are not directly involved in secretion.

Collenchyma cells provide support to young plant parts and are not typically associated with secretion.

Sclerenchyma cells have a primary function in providing mechanical support to the plant, and they often have thick, lignified walls. They are not specialized for secretion.