A permanent plant tissue consisting of thin walled living cells is ___________

The correct option is A Parenchyma
Parenchyma cells have thin walls and are alive at maturity.Parenchyma cells are involved in the storage of starch, sucrose, protein, water, phenol derivatives, many mineral substances, etc. Metabolic activities like respiration, protein synthesis and are also performed by parenchyma cell.

Collenchyma cells are living cells but they have irregularly thickened walls at corners.
Sclerenchyma cells are dead cells with highly thickened walls.
Xylem possesses dead cells with thickened walls.

So, the correct answer is option A.