직장 동료를 꼽은 응답자가 = the respondents who replied "work friend".
누구와 같이 가느냐는 질문에는 직장 동료를 꼽은 응답자가 65%로 가장 많았고,
= To the question of (travel) companion, work mate was the predominant answer, making up 65% of the replies.
꼽다 (base from of 꼽은) = 1) to count as/among (a select group), 2) select/regard as a candidate or prime example of something.
The word has the notion of counting on your fingers, especially for impatiently waiting for a day to arrive. So there is the expression (어떤 날을) 손꼽아 기다리다 (Wait for something counting the days on your fingers), and 손에 꼽는다 (손꼽다 is a closely related verb, likely the original word 꼽다 branched from).
In your example, it's using the word in the #2 sense, "to select".