What is the total number of tetrahedral voids present in a single...

In an fcc unit cell, the effective number of atoms is $Z = 4$.

The number of tetrahedral voids in a close-packed structure is twice the number of effective atoms.

Number of tetrahedral voids = $2Z = 2 \times 4 = 8$.

Alternatively, in an fcc unit cell, tetrahedral voids are located on the body diagonals. There are $4$ body diagonals and each contains $2$ tetrahedral voids.

Total number of tetrahedral voids = $4 \times 2 = 8$.

Answer: $8$