Preheat oven to 375°.
Lightly spoon the flour into a dry measuring cup, and level with a knife. Combine flour, sugars, and salt in a bowl; cut in margarine with a pastry blender or 2 knives until mixture resembles coarse meal. Add peanut brittle, stirring to combine.
Arrange apple in an 8-inch square baking dish; spoon marmalade over apple. Sprinkle flour mixture over marmalade. Bake at 375° for 40 minutes. Serve warm. Top with yogurt.