Ocean acidification is most accurately described by the answer choice that talks about carbon dioxide being emitted into the atmosphere before being absorbed by ocean water to react with carbonate ions and form carbonic acid.

Image: "Ocean Acidification." NOAA PMEL Carbon Program. NOAA. Web. 6 Apr. 2015. <http://www.pmel.noaa.gov/co2/story/Ocean Acidification>.

Red tide is caused by algal blooms, which occur due to eutrophication of waterways. Red tides set off harmful chain reactions in ecosystems, because of the toxins the algae produce. The toxins get concentrated in higher trophic levels (biomagnification).

Since there has been a significant population decline over a substantial time period, but there is still an extensive range and population, this species would best be categorized as "threatened." A species classified as threatened is vulnerable to becoming endangered in the future and is still being impacted significantly by habitat loss and human activities. This frog species would be categorized as endangered if they were at immediate threat of becoming extinct.

Conservationists advocate to manage resources for sustainable continued use by humans. Preservationists are in favor of prohibiting human usage rather than actively managing.

Feral animals often wreak havoc on ecosystems, particularly those that live in large social groups and/or are very destructive, such as hogs.

The United Nations classifies nations as developed or developing based on the nation’s degree of industrialization and per capita gross domestic product purchasing power parity (GDP PPP). A country’s per capita gross domestic product purchasing power parity (GDP PPP) is a measure of the amount of goods and services the nation’s average citizen could buy in the United States. Developed countries include the United States, Canada, Japan, Australia, and most European countries. Most developing countries are found in Africa, Asia, and Latin America.

The tragedy of the commons is a parable that warns of the dangers of mismanaging shared resources. The story tells about herders that share land where their cows/sheep graze. After some time, too many animals are allowed to graze on this shared land which results in overgrazing and soil erosion. Thus the land becomes unsuitable for any grazing animals. This parable can be related to any shared resource such as the atmosphere, oceans, rivers, fish stock, and more.

An ecological footprint is defined as the amount of biologically productive land and water needed to support a person. The per capita ecological footprint is the average ecological footprint of an individual in a given country or area. Most people in developed countries have an ecological footprint that exceeds the environment’s ability to replenish its renewable resources.

The Anthropocene, or Holocene, Extinction, describes the ongoing extinction of thousands of species during the current epoch. A majority of these extinctions are due to human causes, whether through direct overhunting or as a result of pollution and habitat loss. This extinction is expected to intensify over the next century as climate change speeds up and drastically alters many habitats.