why the object in the arrow function is enclosed in ()?

Question

// Write a function that takes two arrays (a and b) as arguments // Create an object that has properties with keys 'a' and corresponding values 'b' // Return the object

function myFunction(a, b)
{
return a.reduce((o,c,i) => ({...o,[c]: b[i]}),{})
// return a.reduce((o,c,i) => return {...o,[c]: b[i]},{}) 
//notworking
// return a.reduce((o,c,i) => {...o,[c]: b[i]}),{} //notworking
}

Test Cases: Attempted: 10598 Solved: 2585 myFunction(['a','b','c'],[1,2,3]) Expected {a:1,b:2,c:3}

myFunction(['w','x','y','z'],[10,9,5,2]) Expected {w:10,x:9,y:5,z:2}

myFunction([1,'b'],['a',2]) Expected {1:'a',b:2}