Just the photo copy.
And this is true - "External Affairs Ministry need to authenticate this document if this document is to be utilized by Foreign missions in India"
This means, if your are using your Marriage certificate in Government of India's department in Outside of India, that's when u need to get attested by External Affairs Ministry. For example, I went to BLS to renew my passport (Republic of India) and told them I need to add my wife name in it and I have Marriage Certificate for proof. They said I need to get it attested by External Affairs ministry or My wife (as she is still living in India) can get my name added in her passport without the attestation and then I can user passport copy to add her name in mine. I know going through External ministry is a pain in the ass. So, my wife added my name first and then I used her passport copy to do mine. That's they mean.
IRCC Canada doesn't ask for it. I assume your marriage certificate is in English. So, I am sure just a photocopy is good.