becoming separate species!! PLEASE HELP

Because they're similar species but slightly different, possibly caused by geographical isolation.

These two species could've started as the same, and then they were geographically isolated so unable to reproduce with each other. The different environments have different selection pressures so different mutations were favoured in different populations, etc. etc.

If the species weren't originally geographically isolated, they would likely have reproduced together, unless of course there was reproductive isolation (such as different mating rituals or incompatible genitalia). It looks like you would get credit for both types of isolation.

Edexcel Salters Nuffield aren't always the clearest when it comes to the questions they ask. I know your answer has already been answered but I understand why you were slightly confused because I did the same exam board (new spec too! ) and in some of their evolutionary questions they do explicitly mention different dietary requirements and habitat whereas here it doesn't so you may have just assumed reproductive isolation.

For SNAB my advice would be to always put both points down because there isn't negative marking. If your unsure you could put something like 'may have arrised from geographical isolation' in brackets - this way you can't lose a mark

P.s. I remember answering that question when going over past papers