What is the species introduced into ecosystems, intentionally or unintentionally, and usually by humans? some can take over - perhaps destroying - entire ecosystems. the lionfish is one example?

These are called invasive species (like the Giant African Snails in Galapagos for example.)

the correct answer is: b. pantala flavescens, because these organisms would have a high level of genetic variation due to sexual reproduction.  

because involves two organisms (parents) whose genetic material combines, sexual reproduction leads to genetic variation in new generations of offspring. the offspring is an unique organism, with new genome, more protected with the higher ability to adapted. this process is fundamental to evolution.