Step 1: Classify wireline logging tools by their physical basis:
Wireline logs are generally grouped by the physical property they measure: electrical logs use current flow and resistivity, nuclear logs use radioactive sources or natural radioactivity, acoustic logs use sound wave travel time, and a few specialised tools use optical or thermal sensing. Placing the FMS correctly in this classification answers the question directly.
Step 2: Examine the working mechanism of the FMS pads:
The FMS tool has multiple pads, each carrying a row of small electrode buttons, that are physically pressed against the borehole wall with a caliper arm mechanism. Each button injects a focused survey current into the formation and measures the resulting micro resistivity of the rock in extremely fine detail, at a resolution fine enough to resolve individual bedding laminations and fractures. This is fundamentally the same physical principle as a normal resistivity log, just applied at a much smaller electrode spacing and higher spatial resolution.
Step 3: Confirm none of the other physical principles are involved:
There is no optical sensor or light source in the standard FMS tool string, so option (A) does not apply. There is no thermal sensing element used to build the image, so option (C) does not apply. There is no radioactive source or gamma/neutron detector in the FMS pad assembly, so option (D), the nuclear approach, does not apply either.
Step 4: Select the single correct approach:
Since the FMS image is built entirely from button electrode micro resistivity measurements, the response mechanism is purely electrical, confirming option (B) as the answer.
Final Answer:
\[ \boxed{\text{Electrical (Option B)}} \]