The suitable preposition to fill in the blank is: among

The complete sentence is: "Mohan distributed the sweets among the children at the orphanage."

Here's why:

• "Among" is used to indicate that something is distributed or shared out to several individuals within a group. It is the most appropriate choice for distributing sweets to multiple children.
• "Between" indicates a relationship between two things or people, which is not applicable in this scenario.
• "For" suggests that the sweets are intended for the children but doesn't specify how they are distributed.
• "In" can be used to indicate location, but in this context, it doesn't accurately convey the act of distributing the sweets individually.