Explanation
Unioning is the correct answer!
From the official documentation:

For example, suppose you have the following customer purchase information stored in three tables, separated by month. The table names are "May2016" , "June2016" and "July2016."

Explanation
As the question mentions, we need to create a SET with the following conditions -> Choose only those customers whose Sales > 30,000
1) Right click on customer name --> Create --> Set

2) Let's Name the Set - Customer>30k ( you can name it anything you want :) ) Select USE ALL, and then move to the CONDITION TAB:

3) In the Condition Tab, Choose BY FIELD -> Select Sales -> Sum -> Greater than 30000 , and click OK

4) You should now have a new Set in the Data Pane as follows:

5) Drag this set to the rows shelf, and click on SHOW MEMBERS IN SET. Now drag Sales to the Column Shelf.

6) Click on the Show Mark Labels, and Sort ascending icons as shown:

7) Voila! We have our answer:
c

Explanation
According to the official Tableau documentation, there are 5 types of trend lines which we can work with in Tableau :
1) Linear Trend Line
2) Logarithmic Trend Line
3) Exponential Trend Line
4) Polynomial Trend Line
5) Power Model
Hence, the correct answer is BINOMIAL trend line which is not present in Tableau.
See the following image:

For more information, refer to: https://help.tableau.com/current/pro/desktop/en-us/trendlines_add.htm

Explanation
As the names suggest, if we subtract the order date from the shipping date, we simply get the number of days between these 2 dates.
We can these use this calculated field in our charts, and can use COUNT, SUM, AVG etc with them according to our need.