Reference:
https://docs.splunk.com/Splexicon:Calculatedfield

The timechart command buckets data in time intervals depending on the selected time range2. The timechart command is similar to the chart command but it automatically groups events into time buckets based on the _time field2. The size of the time buckets depends on the time range that you select for your search. For example, if you select Last 24 hours as your time range, Splunk will use 30-minute buckets for your timechart. If you select Last 7 days as your time range, Splunk will use 4-hour buckets for your timechart2. Therefore, option B is correct, while options A and C are incorrect because they are not factors that affect the size of the time buckets.

https://docs.splunk.com/Documentation/DashApp/0.9.0/DashApp/chartsBub

Explanation/Reference: https://docs.splunk.com/Documentation/Splunk/8.0.3/Knowledge/FXSelectMethodstep

The where command can be used to complete the search below.
... | where failure>successes
The where command is a search command that allows you to filter events based on complex or custom criteri a. The where command can use any boolean expression or function to evaluate each event and determine whether to keep it or discard it. The where command can also compare fields or perform calculations on fields using operators such as >, <, =, +, -, etc. The where command can be used after any transforming command that creates a table or a chart.
The search string below does the following:
It uses ... to represent any search criteria or commands before the where command.
It uses the where command to filter events based on a comparison between two fields: failure and successes.
It uses the greater than operator (>) to compare the values of failure and successes fields for each event.
It only keeps events where failure is greater than successes.