Correct answer is as below. Attributes in the incoming xml payload are always accessed using @.Similarly
*item is required as we have multiple items in the request
%dw 2.0
output application/csv
---
payload.sale.*item map ((value, index) -> {
index: index,
sale: value.@saleId,
itemName: value.desc,
itemPrice: (value.quantity) * (value.price),
item: value.@itemId
} )

Option 1 is the only correct choice due to two reasons. 1) Concatenation is always with ++ sign and not with + sign which makes option 2 and 3 wrong 2) hearders can be accessed with attributes. headers and not with only headers which makes option 4 incorrect

Scatter-Gather sends the event to each routes concurrently and returns a collection of all results. Collection is an Object of Objects. Each object contains attributes and payload from each Mule event returned from a flow. Hence option 4 is the correct answer.

Correct answer is vars."customer"."firstName"
Please you can also access firstName using this syntax vars.customer.firstName.