Dead cells that serve the mechanical function are called as:
A. Sclerenchyma
B. Companion cells
C. Wood parenchyma
D. Collenchyma

Hint: Sclerenchyma tissues are a complex tissue that mainly work as supportive and protective tissues to the plant. These tissues when matured become dead and accumulate high content of lignin which helps in providing strength to the plant.

Complete answer:
There are many types of tissues present in the plants like parenchyma, sclerenchyma, collenchyma, etc. They perform functions according to their role. Sclerenchyma tissues are highly lignified and have high content of cellulose.
Sclerenchyma tissues are two types:
Fibres: Which is elongated, having tapering ends and providing maximum support to the plant. These cells can be found anywhere in the plant body stem, root or leaves of the plant.
Sclereids: These are extremely variable in morphology and present in tissues like pith, cortex, xylem or phloem. These may also present in hard shells or seed coats.

Thus, we can conclude that dead cells that provide mechanical strength to the plants are sclerenchyma tissues.

Note: Cell wall of sclerenchyma can be divided into two types, that is, primary cell wall and secondary cell wall. Primary cell wall is the one having ability to divide and grow that means this layer or cell wall is present in the sclerenchyma tissues when the cells are live and they are not highly lignified. Primary cell wall is less rigid and thinner than the secondary cell wall.
Secondary cell wall develops after the primary cell wall, which contains high amounts of lignin and cellulose which makes it very thick and rigid. And cells are dead in this condition.