https://cloud.google.com/solutions/images/transfer-speed.png

A single vpn can be up to 1.5 Gbps?. Then it would take around 10 hours to transfer 4TB. For 1 time that is maybe acceptable. But what are large updates? 1TB? that would cost 2-3 hours(to much i think)

Then I would prefer Dedicated interconnect.

But large updates from 10GB would take less then 2 minutes and that is okay.

For me this question is not clear enough and still not sure if it is VPN or Dedicated Interconnect

VPN is in public space and interconnect provides private address space

It can't be 'A' because it's not Cloud VPN (https://cloud.google.com/vpn/docs/concepts/overview but a VPN server installed on a GCP instance which not have enough troughput to handle the transfert

Agree with RK, VPN suits well but I would choose D
https://cloud.google.com/vpn/docs/concepts/overview

VPN throughput is @GBits/sec in average: (1.5–3 Gbps for each tunnel https://cloud.google.com/solutions/best-practices-for-iam-and-billing-in-higher-education#networking_options)

So to transfer 1GB = 8Gb requires 4 seconds.
1TB -> 4 * 1024 / 3600 = 1.14h
4TB -> around 5 hours to copy the whole DB

Both VPN and interconnect provide private address space.
I suppose that 1GB per 4 sec are totally enough for large frequent updates of user authentication data into backup replica.

Interconnect requires close location to Google supported colocation facility, either directly or through a carrier and it cost a lot. So there is no reason to make dedicated interconnect just for backup replica.

It is not said that it should be cost effective though. In this case dedictated interconnect is best option.

Something is wrong with your math Alex:
1GByte=8Gbits
transferring of 8gbits via 10 gbits channel will take much less than 18 minutes.
https://cloud.google.com/interconnect/
Assuming that VPN could have up to 3GBs transferring of 4TB would take ~5-12 hours.
So I'd go with VPN if on-premise DC has such hardware.

1 GB om 10 GB network which is min requirement for cloud VPN takes 18 minutes.
1TB -12 days.
4TB -48 days.
Answer C unfortunately is most suitable here.

I agree with Stonewealth, answer is - C
Both VPN and Dedicated Interconnect provide private address space communication. Yet, due to "The database is 4 TB, and large updates are frequent", the Dedicated Interconnect would be able to deliver a more suitable connection bandwidth.

I think answer should be C.