What is the optimal amount of RAM for an ASUS X555LB laptop?

Question

laptops

The laptop has one built-in bracket for 4 GB 1600 MHz in the motherboard, and a second removable bracket with the same characteristics.
It is necessary to increase the amount of RAM and there are such options:
- replace the bar with 8 GB of the same frequency
- replace the bar with 16 GB of the same frequency
- replace the bar with 8 GB with a higher frequency
Which of the options does not make sense, and most likely will not work?
Since the processor is i5 5200u, then the second option will not work either, since 4 + 16 is too much. But in the specification for the laptop there is information that the maximum amount of RAM is 16 GB, although this is not entirely realistic only because of one of the two removable strips. Does it really make sense to put on 16 GB?
Also an option for which it was not possible to find enough information about whether the motherboard will pull

Answer 1

Option A:
with the amendment: "with reduced supply voltage". You have DDR3L wired to your mother.
Why not B?
The specification for your laptop says 12 GB is the maximum (4 + 8 optional).
Why not B?
A tandem of two bars will operate at the lowest frequency among the maximums of each bar supported by the motherboard.
PS: When buying laptops with soldered RAM, you lose the opportunity to replace the bar on the first slot with a bar with max. parameters. So that from the theoretical max. in 16 GB in practice there will be 12 GB.