If the TimeGenerated field is not sent in a call to the data collector API, what happens?

Answer options:

A.A random time will be input into the field based on what is in memory on the server at the time the data in ingested.
B.This field is left blank.
C.A default time of 1900-1-1 00:00 is added to the record.
D.The TimeGenerated field will contain the time the event was ingested via the data collector API.

Correct Answer: D Option D is correct. The name of a field in the data that contains the timestamp of the data item. If you specify a field, its contents are used for TimeGenerated.
Option A, B, C are incorrect. If you don`t specify this field, the default for TimeGenerated is the time that the message is ingested. The contents of the message field should follow the ISO 8601 format YYYY-MM-DDThh:mm:ssZ.
Reference: 

https://docs.microsoft.com/en-us/azure/azure-monitor/logs/data-collector-api