What actually happens to a frame crossing a trunk?
802.1Q inserts a tag into the frame itself. Here's exactly where it goes.
The question
Where does 802.1Q insert its VLAN tag in an Ethernet frame?
The answer
B After source MAC, before type
The four byte tag slots in after the source MAC address, which is why the switch has to recalculate the frame check sequence afterwards.
Why it trips people up
The tag is not a wrapper around the frame — it is inserted into it, which is why the switch must recalculate the frame check sequence, and why a tagged frame is four bytes larger than the original. That size increase is the reason trunk links between switches sometimes need a slightly larger MTU.
Exam tip
Read what the question is actually asking before looking at the options. The wrong answers here are ones candidates genuinely pick — each is true of something adjacent, just not of what was asked.