Since the expansion of international trade, dutch elm disease and chestnut blight have nearly killed off the dutch elm and chestnut trees in america. one might infer that these diseases were the result of

introduced species
competition for space
poor tree husbandry
lack of pollinators

The most appropriate answer would be introduced species.

Introduced or invasive species refers to the organism which is not native to that ecosystem or place and can cause harm to the environment, the economy as well as human health.

So, the expansion of international trade might result in the introduction of Dutch elm disease-causing Ascomycota fungi in America.

Another example may include the introduction of water hyacinth in India.

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.

cell/ tissue/ organ/ organ system/ body