Answer: # Databricks notebook source

The first line of a Databricks Python notebook when viewed in a text editor is a comment that identifies the file as a Databricks notebook source. Python comments are denoted by a `#` symbol, making option C (`# Databricks notebook source`) the correct answer. The other options either use incorrect comment syntax for Python or represent commands that would not appear as the first line in the source file.

Author: LeetQuiz Editorial Team