T. Kajita and A. McDonald, winner of the year 2015 Noble Prize for Physics belong to which of the following countries respectively?

The right answer is C (Japan and Canada). Takaaki Kajita works at the University of Tokyo and led the Super-Kamiokande team, which showed in 1998 that atmospheric neutrinos change flavour, meaning they have mass. Arthur B. McDonald, from Queen's University, Canada, led the Sudbury Neutrino Observatory, which showed that solar electron neutrinos change into other types on the way to Earth. Together they shared the 2015 Nobel Prize in Physics for neutrino oscillations.

Why the others fail:

• A and B pair the USA with Japan, but McDonald is not American, so both are wrong.
• D gets Kajita wrong; he is Japanese, not Canadian.

Concept: neutrino oscillation, which showed neutrinos have non-zero mass, something the original Standard Model did not allow. For Mains, it is a good example of how basic science reshapes theory and links to the Science and Technology section of GS III.

Trap: candidates often guess nationality from the name or assume the Nobel went to a single country. Check each laureate's institution separately, and note that Sudbury is in Canada, not the USA.