In terms of the novel theme why is the moment when Mina gives Van Helsing her transcript of Jonathan journal so important?

The correct answer is that the moment when Mina giver Van Helsing the journal is because is a proof of trust and commitment to keeping a secret. That really trascend on their relationship moving forward.

Explanation:

The journal was from Jonathas time in Transylvania.

When Mina gives Van Helsing her transcript of Jonathan journal she shows commitment, we should remember that she typed from Jonathan's handwriting, and besides it gives Van Helsing the tranquility of being able to trust in Jonathan's word since everything he had written was true.