The experimental group will be the group who received the new weight-loss pill. This group will be used to identify or estimate the effect of the variable or the ingredient which will cause weight loss.
The other group who received the resemblanced of the new weight-loss pill but placebo will only be used for the comparison effect from the experimental group.
So the answer will be :
1st Option, The group that received the new weight-loss pill pack is the experimental group.