Final answer:
The social media manager's task of determining the timing and frequency of content relates to the D)Planning and Publishing pillar of social media marketing.
Step-by-step explanation:
The social media manager's concern about deciding when to post content and how often, in order to engage followers without posting too much, pertains to the five pillars of social media marketing. The pillar represented by this concern is Planning and Publishing. This aspect of social media management encompasses strategizing the content calendar, determining the timing and frequency of posts, and balancing the quantity of posts to maintain impactful engagement without overwhelming followers.