Step 1: Relevant provision.
Under the Indian Designs Act, 2000, the initial term of registration is 10 years from the date of registration.
Step 2: Extension clause.
This initial term can be extended only once, for an additional 5 years, upon payment of the prescribed fee before expiry.
Step 3: Elimination of wrong options.
(a) Incorrect — 20 years is not the base term.
(b) Incorrect — extension is for 5 years, not 10.
(c) Incorrect — multiple extensions are not allowed; only one extension is permitted.
Step 4: Conclusion.
The correct protection period is 10 years + one extension of 5 years.
\[
\boxed{(d)}
\]