Learn English – “Defense” or “defence”

The use of practice in practice problems is correct. Here, practice problems is a compound noun with problems modified by practice. It means problems used for practice.

It helps your understanding to replace practice here with revision. We use revision problems, not revise problems, to mean problems used for revision. Practice is simply a noun meaning the act of practising, just like revision is a noun meaning the act of revising.

There is no difference in meaning between offence and offense. They're exactly the same in all their definitions. The difference is that offense is the preferred spelling in American English, while offence is preferred in British, Indian, Canadian, and Australian English.

These American publications use offense:

The veteran tight end never found a home in Mike Martz’s offense and was inactive for all but five games. [Chicago Sun-Times]

If people take offense at hackneyed phrases it’s because they’re hackneyed . . . [The Atlantic]

And these non-American publications use offence:

Both offences can exploit some areas that play to their strengths. [CBC]

Parents who fail to keep air guns away from their children will be fined up to £1,000 under a new offence from next month. [Telegraph]

Pulpit choice gives offence [Sydney Morning Herald]