Concept:In English, some verbs combine with fixed prepositions to form phrasal verbs with a specific meaning.
Here, the correct phrasal verb is "stand by", which means to support or be loyal to someone.
Explanation:The sentence expresses a promise of continued support: "I will always stand ...... him."
The phrase "stand by" exactly fits this idea of being loyal and supportive.
So the completed sentence is: "I will always stand by him."
Option B, "with", can sometimes suggest agreement or being alongside, but it is less idiomatic for lasting loyalty.
Options C and D do not form the intended meaning of support.
Therefore, the best choice is option A, "by".
Answer:A. by