Choosing a subject to drop follows a similar approach to choosing a subject to take. When selecting subjects, students usually consider elements such as their interests, their performance potential, and the relevance of their subjects to their studies beyond the HSC (if applicable). When dropping a subject, students may wish to consider the subject that they have not enjoyed/have enjoyed the least compared to their other subjects and/or their lowest-performing subject. If the subject has no relevance to their studies beyond the HSC, then this could also inform their decision to drop the subject.
In your case, you mentioned that you are performing well across your subjects. This essentially indicates that you are capable of maximising your ATAR, regardless of the specific subject that you end up dropping. Based on this, you may wish to consider the subject that you enjoyed the least out of Agriculture, History Extension or Mathematics Standard. Generally speaking, students are more likely to maintain their commitment to subjects that they enjoy. Additionally, you should consider whether any of those three subjects may be relevant to your studies beyond the HSC. If so, you should probably keep the subject.
Another element to consider is unit value. You mentioned that you want to drop one subject, meaning that you would end up with a different number of units depending on the subject that you end up dropping. For example, if you drop Agriculture or Mathematics Standard, you would end up with 11 units, whereas if you drop History Extension, you would end up with 12 units. This is important to consider because 1 backup unit can only halve the negative effect of unfavourable performance in another 2 unit subject on your ATAR, whereas 2 backup units can completely negate the negative effect of unfavourable performance in another 2 unit subject. You should therefore consider whether 1 backup unit is enough for you, or whether you should opt for 2 backup units.
I hope this helps!
