ANS:D - teak.

Teak wood is used for railway sleepers.
I think the answer can be Shisham which yields hardwood. Teak --> most valuable timber (age 40years).
Shisham--> hardest wood but not most valuable! Teak wood is most valuable as it is used in furniture.
For railways sleepers generally, deodar is used.
& shishum strongest among all.