Basket of carrots is a quest item in the Blood and Wine expansion that can be found during the main quest, The Beast of Toussaint.
After approaching the "unicorn" you need something to lure it and this, along with the ripe apple and fragrant cake, can be looted from the various picnics nearby. This will work, as well as the ripe apple (or by using Axii, though that'll lead to a fight with the other participants).
The item appears under the "Weapons" tab.