All three task rewards are E-level, specifically, they are between E-level and D-level.
'Mercenary' is an E-level Talent Sequence, but it came with a Secret Medicine recipe for a D-level Talent Sequence. 'Mechanized Body' is also an E-level Mechanized Body, however, it came with a laser cannon that could harm D-level, and even C-level targets.
The third option related to Martial Arts was actually similar, it came with a strand of 'Qi' that one would only possess upon breaking through to D-level.
As usual, first rule out Mechanical Modification.
Although Mechanized Body seemed to be the strongest among these three options, capable of harming up to C-level, Mechanical Modification was not realistic until qualified mechanized body repair tools and replaceable parts appeared in the main world.
Ultimately, Mechanical Modification relied too heavily on external technology, with no clear path to self-improvement in sight.