There is at least one reference mentioning that FTP is not supported in AppEngine: https://cloud.google.com/appengine/docs/standard/java/sockets/

Now I know this is language-specific, but it still strongly suggests that this would probably not be a good idea. This leaves us with GKE+BigQuery --> option B (bottom left), answer C .

Check question 20 and 37 to see if it helps you with this question.
It is pretty much clear that BigQuery is the option to be chosen (see https://cloud.google.com/storage-options/) use BigQuery for Analytical reporting on large data and Data Science and advanced analyses. That let us between the option in the left, being:
Option 1: LB-GKE-Pub/Sub-Dataflow-BigQuery-Analytics
Option 2: AppEngineFlex-Pub/Sub-Dataflow-BigQuery-Analytics
Gary told us FTP is not supported I couldn t find anything other than a question posted in stackoverflow forum. Anyway I like the fitst option better I would go option 1 (top-left image)

I go with "D. Option A"

Agree with Robert. Answer C is the most obvious one for being correct. FTP (Unencrypted data transfer) is not supported by any GCP service, hence you need to use sftp which is supported by gcloud utility which can be run from an AppEngine. By deduction Cloud SQL is not supporting the amount of data captured hence we need BigQuery. Option B --> Answer C

D - Option A, FTP is not supported by App Enginee

C - Option B is the good answer (the bottom left picture) so the right choice is C