To stick to one's guns

Sticking to one's guns means not changing and keeping one's stand when faced with opposition or counterarguments. 

Of the given options, A comes closest to capturing the idea of this phrase. 

Option B: It's not necessarily suspecting but keeping one's original opinion. Hence, this option can be eliminated. 

Option C: This, too, is unrelated to the interpretation of the phrase. 

Option D: The phrase does not mean to be satisfied with your own understanding but rather to stand your ground when argued against. 

Therefore, option A is the correct answer.