He now had a perfectly good reason to believe

[EngLearner]

1. He now had a reason to believe, based on what he'd heard from journalists and fellow businessmen, that he might be killed.

2. He now had a good reason to believe, based on what he'd heard from journalists and fellow businessmen, that he might be killed.

3. He now had a perfectly good reason to believe, based on what he'd heard from journalists and fellow businessmen, that he might be killed.

In which of the above versions is the indefinite article "a" optional/required, and in which of them should it be omitted?

 

[jutfrank]

There's no difference between them in that in all three you could use either version, with or without the article. The only difference between the sentences is the modifiers (good and perfectly).

Without the article, reason is uncountable, whereas with the article it's countable.