Suppose we have a collection, "Students", with one document preexisting:
{ "_id" : 100, "name" : "Student1", "age" : 12, "subject" : "biology" }

Which of the following statements would set the available field to 1?

The right answer is Answer 2.

The query part, {_id:100} identifies the document we want to update, and the update part, {$set:{available:1}}) sets the "available" field to 1.

db.Students.update({},{$inc:{available:true}}) is wrong because $inc only works on numbers, and true is a boolean.

db.Students.update({_id:100, $set:{available:1}}) is wrong because update expects (at least) two arguments: a query part and an update part.