Correct answer is below as it follows the correct syntax.
/customers:
/{customerId}:
get:

Correct answer is Array.
Flatten turns a set of subarrays (such as [ [1,2,3], [4,5,[6]], [], [null] ]) into a single, flattened array (such as [ 1, 2, 3, 4, 5, [6], null ]).
This example defines three arrays of numbers, creates another array containing those three arrays, and then uses the flatten function to convert the array of arrays into a single array with all values.
Source
%dw 2.0
output application/json
var array1 = [1,2,3]
var array2 = [4,5,6]
var array3 = [7,8,9]
var arrayOfArrays = [array1, array2, array3]
---
flatten(arrayOfArrays)
Output
[ 1,2,3,4,5,6,7,8,9 ]

The APIkit Router is a key message processor that validates requests against the provided definition, enriches messages (for example by adding default values to the messages) and routes requests to a particular flow. Also, the Router raises errors messages if errors occurs while routing, validating or processing the user request.

Setting Breakpoints
To set breakpoints, right-click a building block, then select Toggle Breakpoint.
Studio applies a red dot to the building block's icon on the canvas.
When you run your application in Debug mode, Studio stops the flow execution at the breakpoint you have set, allowing you to check the Mule Event content in the Mule Debugger View.
Mule Ref Doc : Setting Breakpoints | MuleSoft Documentation