Which is biotic?
a. wind
b. iron gate
c. sunlight
d. dead leaf

The most appropriate answer would be d. dead leaf.

Biotic component of the ecosystem includes all the living things such as plants, animals, and microorganisms.

Leaf dead or alive should belong to the biotic component of the ecosystem as it was once the part of the plants.

It can be co considered as abiotic once it is completely degraded or broken down into its simpler constituents.

explanation:

b,bacteria convert nitrates

into nitrogen gas.

d,plants and animals take in

and use nitrogen compounds.

e,bacteria break down waste    

products and return nitrogen  

to the soil.

a,bacteria convert atmospheric

nitrogen to ammonia.

c,bacteria convert ammonia

to nitrites and nitrates.