Tolong is used when the say-er needs a favor. If you have fallen with your bike, you use tolong to ask for help. So in this Duelingo example Tolong is technically wrong. Silakan is used when you want to say please but it is not in your favor. So in this example Silakan makan, would be perfect, or Mari makan. Mohon is used to apologize or aks for permission. Mohan maaf mbak, saya salah. Note: silakan is without a "h".