An Architect has configured a SAML-based SSO integration between Salesforce and an external Identity provider...

 these are the optimal actions to troubleshoot a SAML error. According to the Salesforce documentation1, you can use the following methods to debug a SAML error:

Use a browser that has an add-on/extension that can inspect SAML. This will allow you to see the SAML request and response messages and identify any issues with the SAML assertion or the SAML response2.

Paste the SAML Assertion Validator in Salesforce. This is a tool that helps you validate the last SAML operation on your organization and shows you any errors or warnings with the SAML assertion or the SAML response1.

Option A is incorrect because the Callback URL is not related to SAML SSO. The Callback URL is used for OAuth SSO, which is a different protocol3. Option D is incorrect because using the browser’s Development tools to view the Salesforce page’s markup will not help you debug a SAML error. The page’s markup doesnot contain any information about the SAML request or response4.