"When A mosquito bites on our leg, Our brain sends a signal to the hand to kill the mosquito on that particular position of the body. And it can be all done within miliseconds."... I think that can be a good example for the situation.

1st line: Our actions follow our thoughts. we all believe so.. Which extrapolate: Thought = Action.

2nd line: "Every thought, However ___, causes muscular action." ...This part is pretty similar to our extrapolation(thought=action). So for the blank, we need to search for the word which defines our thought.

3rd line: "which leaves its trace in that part of the physical organism which is most closely allied to it."
Here, This sentence clarifies the action in detail.

Now we look for options
A) transitory..... which means "fleeting or lasting a very short time"
-- Thought could for short time. Besides, "However" adds extra support to the word and gives a negative flavor. That sounds perfect with the whole scenario.
B) important... meaning "great significance or value.
It doesn't add any negative flavor as per "However". So it's not a good choice.
C) pertinent---Which means "relevant or applicable to a particular matter"
Since, thought=action, Which means, it doesn't matter how relevant or irrelevant our thought is, the body will act spontaneously. So, This option would be the wrong choice.
D) amenable----means "susceptible" or "easily persuaded or controlled".
As per our extrapolation Body organs are susceptible to our thought. So it doesn't fulfill our extrapolation.
E) loquacious--- which means "talkative!!!!!!!!" Definitely 100% out of scope.