Hi Suwarna,
Basic SKU load balancer does not support Availability Zones. You will need to use Standard SKU if you need that.
Azure Load Balancer SKUs
https://learn.microsoft.com/en-us/azure/load-balancer/skus#skus
Please see article below for more information regarding using load balancer with availability zones:
Load Balancer and Availability Zones
https://learn.microsoft.com/en-us/azure/load-balancer/load-balancer-standard-availability-zones
Please click Accept Answer if the above was useful.
Thanks.
-TP