The Principle sorts every plea of "I did not know" into one of two boxes. Ignorance of fact goes in the box marked excused. Ignorance of law goes in the box marked no excuse. Answering this question is entirely a matter of putting X's plea in the correct box, and the facts make the sorting easy.
What was X ignorant of? Not the gold, it was on his person and he must have known 34 kg of bars were there. Not the Manifest, since he knew he had not entered the gold in it. The only thing he did not know was that a notification dated 26 November had removed the earlier exemption and made declaration compulsory. A notification is law. His plea is therefore ignorance of law and it goes straight into the box marked no excuse. He can be prosecuted.
The two day gap is the trap. It feels harsh that a rule two days old can catch a traveller who had no chance to hear of it. But look at the Principle again and you will not find any words about how long a law must have been in force. It is unconditional. If newness were a defence, every fresh rule would be dead law for some undefined period, and no one could say how long.
Let's summarize:
The correct answer is option (C).