It spreads tasks across Availability Zones, then distributes tasks evenly across instances within each zone is correct.

ECS evaluates placement strategies in the order they are listed and applies the first rule then the next. A spread on attribute ecs.availability-zone balances tasks evenly across Availability Zones and a subsequent spread on instanceId balances tasks across container instances inside each zone so tasks end up distributed evenly across instances within each zone.

It spreads tasks across Availability Zones, then randomly assigns tasks to instances inside each zone is incorrect because the spread strategy creates even distribution across the chosen dimension rather than performing random placement.

It spreads tasks across Availability Zones, then packs tasks based on lowest available memory per zone is incorrect because packing by memory or CPU requires the binpack strategy rather than spread.

It spreads tasks across Availability Zones, then guarantees each task lands on a different instance within the zone is incorrect because guaranteeing one task per instance requires a placement constraint such as distinctInstance rather than using spread.

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