Mainly (b) and (c), Hypo.
Many migrants work in businesses where they are paid cash in hand, often below the minimum wage which leads us in to (c).
Many of the jobs taken by migrants either pay very low rates of pay (usually the minimum wage or sometimes less) or involve a low number of hours per week or both. Someone working 30 hours a week at the minimum wage (a fairly typical arrangement) will earn roughly £9,000 pa (depending on how much holiday pay they receive). On this he will pay no income tax. He will pay about £250 NI and his employer will pay about £300 NI. Having paid his £250 he will be entitled to about £780 in working tax credits. He will almost certainly be entitled to Housing Benefit and if he has any children (either here or abroad) at the very least he will be entitled to Child Benefit (£1,066 pa for the first child, £705 pa for each subsequent child). Let's assume he pays about £1,000 in VAT and excise duty on items he buys (this is conservative as he probably sends most of his spare cash "home"). So, contributions of £1,000 (VAT/excise) and £550 (NI) will be offset by £780 working tax credits, making him a contributor to the tune of £770. However, his HB will almost certainly wipe that out entirely, and some. And remember, this is assuming he has no children.
The sums, although rough and ready, are not too unbelievable. Of course this individual, who almost certainly makes no net contribution to the tax pot and is probably a hefty net beneficiary depending on his exact circumstances, will be entitled to all the services the NHS provides. That is why it is essential to consider the effect that mass migration of people doing low skilled low paid jobs has upon funding for the NHS.
Of course it could be argued that somebody who already lives here would produce a similar calculation. That would be a valid argument if there were no people unemployed. But whilst we are importing labour in the form of people who are net beneficiaries of the tax system we are also paying around 2m people to sit at home because they will not do the work the migrants do. And that's why the country's finances are in a state.
It ain’t rocket science. You just need to do a few sums before you blandly accept that mass migration is good for the country.