Which of the following living cells provides the tensile and mechanical strength?

The correct option is B Collenchyma
Collenchyma tissues are the simple tissue which is primarily composed of similar type of cells which are irregularly thickened at the corners. These provide tensile and mechanical strength to the plant.
Sclerenchyma cells have thick lignified secondary walls and often die when mature. Sclerenchyma provides the main structural support to a plant. Sclereids are a reduced form of sclerenchyma cells with highly thickened, lignified cellular walls that form small bundles of durable layers of tissue in most plants. The presence of numerous sclereids form the cores of apples and produce the gritty texture of pears. Phloem is involved in transport of food and not in mechanical support.

Thus, the correct answer is option A.