Let's say you want to see if another star goes through a starspot cycle like our Sun goes through a sunspot cycle. To be sure it is cyclic activity, you should watch the star for two cycles. If the other star's cycles last as long as our Sun's sunspot cycle, how long should you study the star?
about 22 years

the wrong statement is - c. a desert had more biodiversity than a marsh.

the deserts are one of the places of the planet where there's the least biodiversity. the reasons for that are the climate and the lack of food sources, which hampers both the number of the species that can survive in it, and the number of the individuals in the species itself.

the marshes, on the other hand, are places where there's perfect conditions for lots of organisms to thrive. the temperature is moderate and nice, the amount of water and the food sources are on a high level, so there's an abundance of both plant and animal life in them.