Im stuck on this question Which of these ecosystems do you expect to have the highest productivity?

The image on the bottom right appears to be some kind of moist forest and compared to the others it will be the most productive.

Bottom right...........

They appear to be arranged left to right top to bottom.

Lowest productivity: upper left, hot desert. Too arid for much productivity. Plants grow slowly.

Higher: upper right, tundra. Short growing season. Trees don't grow there because of permafrost.

Higher: lower left, taiga / boreal forest. Spends too much time with snow on the ground.

Highest: lower right, broadleaf forest. Longer growing season, deciduous trees, brush.

Not pictured:

Tropical rain forest. These would be the highest productive terrestrial ecosystems of all. Warm all year 'round, never frozen, water available all the time.

My guess would be the pond.