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のが本当の

is the real

→ の
phrase Rare #32,645
is the real の turns the preceding phrase into “the fact that …”; が marks it as the subject. 本当の means “true” or “real” and is followed by a noun.
JA: 毎日続けているのが本当の強さだ。
EN: The fact that you keep going every day is real strength.

本当に好きなのが本当の理由だ。

The real reason is that I genuinely like it.

本当に楽しかったのが本当の思い出だ。

The true memory is how much fun I really had.

本当にうれしいのが本当の気持ちだ。

The truth is, I'm really happy.

本当に困っているのが本当の状況だ。

The truth is, I'm really in trouble.

本当に感動したのが本当の感想だ。

The truth is, I was genuinely moved.