What is the best aero position in a mountain bike?
Caveat: As mentioned by @R.Chung if you are uncertain, it is best to seek clarification from your coach.  You are after all paying for their services.  

Short Answer

Bike positioning is always a compromise between goals, flexibility and ability. There are different aero positions and each will have different strengths and weaknesses (see my suggestions section).  I suspect the exact aero position isn't too much of a concern to your coach, rather s/he is likely interested in getting you to do  efforts in a more closed position to get you prepared for riding in an aero position during a race.

What follows is a more detailed explanation.

Why train in an aero position for XC racing?

As previously stated, I suspect the coaching exercise is to get you used to working hard in a closed body position. All aero positions will get the trunk of your body lower (to reduce your frontal area, making you more slip  through the air with less effort). Similar to road racing,  XC racing can also be a game of marginal gains.  If you can cut more drag on long flat sections than another rider, you might be able to pick up a position or maintain your positioning with less effort.

While aero positions can be useful in a race, a closed body position also affects your pedal stroke and your sustainable power output (typically reduced power) as you will be recruiting your muscle groups differently due to a different hip and back posture than you spend most of your riding hours in.  If you never train in this body position, it won't be an efficient position for you to ride in come race day.  Furthermore, spending time in this position it will also require you to hone your the stabilizer muscles needed to maintain that position.

In all likelihood your coach is trying to give you some training time in this type of position.  The exact aero position chosen will likely not matter too much, as long as you have a closed body position so you can train muscle recruitment and stabilizers.

Some Suggestions

My suggestion (confirm with your coach) would be to try a variety of positions (the two you found pictures of is a good starting point), but stick to one aero position per interval effort.  Then in each successive effort try a different posture. This will let you get a feel for how your body works. 

In the two pictures you included, the first aero position one will have the lowest frontal area (aka fastest) as your arms are tucked in, but this will make the bike less stable and is likely not a good choice for rougher terrain.  The second position you show sacrifices some frontal area for stability.  Both have a reduced trunk angle, so both will work on your ability to recruit pedalling muscles while in a closed trunk position.

As a side note, you can also use the training time to find the best aero body trunk angle.  If you go too low you will lose more power than you save in reduced drag, too little of an angle and you may still be working too hard against the wind. Keep an eye on the transect between power (if you got it), speed and heart rate to try and find the best compromise for your current flexibility and training.

The "right" compromise is very rider specific and can change over time with training.