Because the answer to this question depends heavily on which brain region is stimulated, I will focus on the visual cortex. Electrodes implanted in the primary visual cortex (V1) can generate visual percepts (flashes of light, or phosphenes) in the blind. Specifically, cortical prosthetics placed on V1 can generate phosphenes in the blind, and have been successfully used to restore partial vision in the blind (Dobelle, 2000). The generation of phosphenes is then the 'action' you are referring to in your question.
However, it all depends on stimulation conditions whether phosphenes are actually generated; most notably the stimulus level (but also stimulus shape (waveform), stimulus duration etc.). Assuming there is above-threshold stimulation in the cortex, a noticeable phosphene will be evoked. So this would probably be categorized as option 2. The triggering of a subject's "will" is probably very difficult, as triggering a voluntary choice by external stimulation has not been done at this time, as far as I know.
In case of a just-above threshold V1 activation, a subject may see a phosphene on one stimulation, but not the next. Blind folks often have spontaneous activity in their visual system, which makes them see spontaneous random patterns of light, or even complex scenes (Ayton et al., 2015). In this case, a phosphene may be generated, but go unnoticed by the subject, which boils down to option 1 and proves option 3 and 4 to be untrue.
So on the basis of the example scenario of a visual prosthetic for the blind, options 1 and 2 are correct, dependent on whether the stimulus is far above threshold, or just above it. Options 3 and 4 are incorrect as they are incompatible with the first two options.
My answer can be translated to a (hypothetical) cortical implant on the motor cortex in, e.g., people suffering from paralysis or motor dysfunction due to, e.g., stroke or trauma. Also, transcranial stimulation of the motor cortex is a good example, where intense stimuli generate limb movements that are seen and felt by the subject, but near-threshold stimuli may evoke movements that may go unnoticed due to spontaneous muscle movements.
- Ayton et al. Ophthalmology (in press)
- Dobelle et al. ASAIO (2000), 46(1): 3-9