What does it mean by the youngest religion? Is it the oldest or most recent (used today) religion?

Young religions, also called New religious movements (NRMs) are religions created recently, examples like LaVeyan Satanism (1966), Thelema ( early 1900s), Latter Day Saints (LDS / Mormons, 1820s), Jehovah's Witnesses (JWs, late 1870s), Discordianism (1963)

Older religions include Judaism and Hinduism, etc

"Most recent" is difficult to answer, but the largest religion today is Christianity with over 2 billion followers, followed by Islam and then irreligion.

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.