What type of invertebrate are robber crabs? why do these crabs fit in this category?

The are arthropods. They are arthropods because they undergo ecdysis, have segmented bodies and the key trait of jointed limbs “arthro”=jointed and “pod”=foot

youre right there are

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.