Why are marshes more productive than bogs? a. marshes have stagnant water. b. marshes get more sunlight. c. marshes are larger in size. d. marshes have more water flow.

Option D, Marshes have more water flow.

Explanation:

Marshes have water flowing round the year and thus they have wetlands which are nutrient enriched while bogs have stagnant water and thus they have poor soil with low nutrients and high content of peat.  

Marshes being nutrient enriched support a wide variety of reeds, grasses and woody trees while bogs have only mosses, fungi and shurbs  as their fauna.

Hence, option D is correct