The man suggested the groat was among the legal tenders being considered by nationalists, leaving Joanna Cherry, MP for Edinburgh South West, speechless.
He said: “What currency are you going to use if you get the referendum and you leave the United Kingdom what currency have you got?
“The Groat is it? The pound or the Euro?
“Which currency are you going to use?”
Watch the clip, above.
Cherry, who is the party’s Justice and Home Affairs spokesperson, was at first lost for words, before she said: “It’s perhaps worthwhile reminding ourselves that viewers in Scotland are watching this programme tonight and we perhaps shouldn’t ridicule them as I wouldn’t wish to ridicule people in the audience.
“To answer your question, sir, unlike the British government, when we hold our next referendum in Scotland we will have a detailed plan.
“We will set out our plan about how we propose to secure Scotland’s place in Europe.
“We will set out our plan to ensure a rosy economic future for Scotland.
“And we will set out our plan for the currency then, we are working on that at the moment.”
Scots groats were issued between the 12th Century and the 1850s.
It comes after Theresa May brushed Nicola Sturgeon’s moves towards a second Scottish independence referendum aside.
May said on Thursday “now is not the time” for a potentially divisive referendum on Scottish independence - citing the potential impact on Brexit negotiations.
The Prime Minister is expected to trigger Article 50, the formal process of leaving the EU, in the next two weeks. Negotiations will last a maximum of two years.
Thursday’s Question Time was filmed in Bognor Regis - and was beset by claims one audience member was a “plant” due to their critical views of Labour leader Jeremy Corbyn.