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Sacrifice a Food: Target creature blocks this creature this turn if able.
If an effect refers to a Food, it means any Food artifact, not just a Food artifact token. For example, you can sacrifice Tough Cookie (an Artifact Creature — Food Golem) to activate Maraleaf Rider's ability (an ability with "Sacrifice a Food" in its cost).
If the target creature can block other creatures but can't block Maraleaf Rider, it's free to block any other creature or to not block at all.
If the target creature can't block for any reason (such as being tapped), then it doesn't block. If there's a cost associated with having it block, its controller isn't forced to pay that cost, so it doesn't have to block in that case either.
