Concept:Sunshine duration is measured by instruments that record the amount of bright sunlight in a day.
Explanation:The Campbell-Stokes recorder focuses sunlight through a glass sphere onto a card.
The focused sunlight burns a mark on the card, showing how long the sun shone.
A cup-anemometer measures wind speed, not sunshine.
An aneroid barometer measures atmospheric pressure, not sunshine.
Therefore, the correct instrument for measuring duration of sunshine is the Campbell-Stokes recorder.
Answer:A. Campbell-Stokes recorder