Apex domains cannot have CNAME records.
Question and response
Question
dig airtelbroadband.in. NS
Answer
airtelbroadband.in. 3 NS dnsdel.mantraonline.com.
airtelbroadband.in. 3 NS aaadel.mantraonline.com.
Authority
The response has no authority records.
Additional
aaadel.mantraonline.com. 5 A 202.56.230.6
dnsdel.mantraonline.com. 5 A 202.56.230.5
aaadel.mantraonline.com. 5 AAAA 2404:a800:0:b:0:0:0:9
aaadel.mantraonline.com. 5 AAAA 2404:a800:0:14:0:0:0:3212
dnsdel.mantraonline.com. 5 AAAA 2404:a800:0:b:0:0:0:9
dnsdel.mantraonline.com. 5 AAAA 2404:a800:0:14:0:0:1:1212
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0