Name the mathematician whose portrait is on a £50 bank note.

The portrait on the £50 bank note currently features the celebrated mathematician and logician, Alan Turing. Turing is renowned for his pivotal role in the development of computer science and artificial intelligence, as well as his contributions to cryptography during World War II, particularly in breaking the Enigma code, which had a significant impact on the war's outcome.

Alan Turing's legacy extends beyond his immediate contributions; he has become an iconic figure for his brilliance and for the injustices he faced during his lifetime, making his image a poignant symbol of progress in both technology and human rights. The decision to feature Turing on the bank note reflects a recognition of his extraordinary achievements and the importance of diversity in celebrating the contributions of individuals throughout history.

The other options, while prominent figures in mathematics, are not the ones depicted on the current £50 bank note. Isaac Newton, for instance, is honored on the £1 note from the past, and Charles Babbage is also a significant historical figure, particularly for his work on the mechanical computer, but he is featured on the £20 note. As for Euclid, he is often referred to as the "father of geometry," but he does not appear on any contemporary British banknotes.