Verified answer

It's the first one.

It's always きゅう when you say numbers by themselves.

When it is linked to something else, sometimes it becomes く. For example 九月　「くがつ」.

You just have to learn them.

Forty nine in Japanese.

Both of them are fine.

よんじゅうきゅうYonjukyuu (pronounced like ywon-jue-quewoo)

よんじゅうく Yonjuku (pronounced like ywon-jue-k)

Both of them sound natural to Japanese.

But,

In case you want to mean USD 49, you should say "よんじゅうきゅう doru".

In case you want to mean JPY 490,000 "よんじゅうきゅう 万円(man' yen)".

Sorry I am not a professional language teacher.

And, I am not an English native speaker, I am sorry if my sentences make no sence.