A physical feature, such as an ocean, which hinders migration is an example of - an environmental incentive.
- an environmental push factor
- a forced migration.
- an intervening obstacle.
- a cultural pull factor

I would think an environmental incentive - i would say a cultural pull factor but this has nothing to do with culture, the migration isnt said to be forced and it wouldnt be intervening or push because its as stated in the first - an incentive for migration