To answer this question, we need to understand the difference between "affect" and "effect".
The word "affect" is usually used as a verb and means to have an influence on or to produce a change in something. On the other hand, "effect" is usually used as a noun and means the result or outcome of something.
In the given sentence, the word "effect" is used correctly. It states that "his opinion had no effect on my decision." Here, "effect" is used as a noun to describe the result or outcome of his opinion on the decision being made.
Therefore, the correct answer is A) True.