An enveloped virus is dependent on the envelope for its ability to infect a host cell. which virus most likely uses this mechanism?
hiv
polio
rabies
ebola

The right answer is HIV virus.

The HIV has on its surface about twenty trimers gp120 / gp41 (on its envelope) which will be used for the attachment of the virus on a cell receptor (CD4) of the target cells. These two proteins are encoded by the same gene (Env) and are synthesized as a precursor which will be glycosylated (addition of sugars) and cleaved by a protease.

Fixation of the trimer to the CD4 receptor induces a conformational change in the gp120 protein, which will increase its affinity for cellular coreceptors (CCR5 or CXCR4). Again, a conformational change occurs on the gp41 protein, which will allow the fusion of the virus and cell membranes.

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.