Can you native grammarians teach me which is the correct answer and why?

D is correct:

Employees attending the Expo are welcome to travel to the event site once they choose. (...from the options available)

C is okay but a little clumsy:

Employees attending the Expo are welcome to travel to the event site where they choose. (...to go)

I'd rephrase it:

Employees attending the Expo are welcome to travel to the event site of their choice.

Employees attending the Expo are welcome to travel to the event site of their choice. Can you rephrase your sentence into the following?
Employees attending the Expo are welcome to travel to the event site (which) they choose.
What do you think of "Employees attending the Expo are welcome to travel to the event site afterward (which) they choose."?

A is correct.

This works and it is not incorrect. It sounds a little "off" (to me) but that could be because it's isolated from its context.



Again, this is removed from its context. I think sounds better this way:

1. Afterward, employees attending the Expo are welcome to travel to the event site which they choose.

or

2. Employees attending the Expo are welcome to travel to the event site afterward which they choose afterward.

And you can actually leave out the "which":

3. Employees attending the Expo are welcome to travel to the event site they choose afterward.

Here's an example I found:



Now read this:

4. You know that little Italian restaurant - the one I mentioned in my letter?

"Which" is implied and needn't be written. I hope this isn't confusing — I'm glad I grew up speaking English and didn't try to learn it as a second language!

The purpose of the statement is to inform the employees that they are welcome to travel to the Expo in whatever way they choose . This refers to means of travel. Therefore, D makes no sense. The answer is A.

I understand this to mean that there are multiple event sites at the Expo and employees have the choice to visit any of them. Therefore A makes no sense.
That's what I mean when I point out the lack of context. As an isolated statement it can be interpreted in different ways.

Accounting for lack of context, and taken in the context that you recognize, it should read: "Employees attending the Expo are welcome to travel to the event site of their choice."

Taken in the context that I recognize, the Expo is the event site, and therefore it would read: "Employees attending the Expo are welcome to travel to the event site however they choose."

It's A but the sentence is a bit ambiguous. The missing word refers to the means of travel. There's no evidence of more than one event or site, from the words we've got in front of us.

I stand corrected. But I don't think the sentence is that clear to begin with. I thought multiple events were implied because that's what you usually find at an "Expo".

It's definitely an ugly sentence!