Apex domains cannot have CNAME records.
Question and response
Question
dig claro.com.sv. NS
Answer
claro.com.sv. 5 NS ns2.navegante.com.sv.
claro.com.sv. 5 NS ns1.claro.com.sv.
claro.com.sv. 5 NS ns4.navegante.com.sv.
claro.com.sv. 5 NS ns1.claro.cr.
claro.com.sv. 5 NS ns1.navegante.com.sv.
claro.com.sv. 5 NS ns3.navegante.com.sv.
Authority
The response has no authority records.
Additional
ns3.navegante.com.sv. 5 A 201.247.155.225
ns4.navegante.com.sv. 5 A 201.247.157.225
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0