Read this: In the United Kingdom, a bill is presented for royal assent after it has passed all the required stages in both the House of Commons and the House of Lords. Under the Parliament Acts 1911 and 1949, the House of Commons may, under certain circumstances, direct that a bill be presented for assent despite lack of passage by the House of Lords. Officially, assent is granted by the sovereign or by Lords Commissioners authorised to act by letters patent. It may be granted in parliament or outside parliament; in the latter case, each house must be separately notified before the bill takes effect.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): Who can be authorized to grant assent by letters patent?
the sovereign or by Lords Commissioners