I would use the “Stereo DVD” option.
AGC stands for Automatic Gain Control which reduces the volume if the signal is strong and raises it when it is weaker.
I don’t know about you but I don’t want my carefully prepared audio tracks messed with by some auto circuit.
FYI, my guess is that you’re using Movie Studio. If I’m mistaken and you’re using Vegas Pro, then you should be using the Dolby Digital AC-3 Pro option as shown below.


If you’re concerned about your audio levels not coming out the way you think they should, modify the preset like so:
Encode set to AC3;
Click on Custom tab;
On the first tab, set Dialog Norm at -31 (this sets it at unity gain);
On the last tab marked Preprocessing, set the Line Mode Profile and RF Mode Profile mode to “none”;
DC High Pass Filter On (this operates at <=3Hz and takes DC bias out of the mix);
Bandwidth Low Pass Filter On (this takes aliasing noise at >20kHz out of the encode).
Save this as a preset.