iPhoneX: Which Apple 29W, 30W, 61W, or 87W USB-C Power Adapter?

Hmm, well the only things that comes to mind are:

1. The rep is new or lack sufficient training to answer the question.
2. The rep didn't have their coffee (or favorite beverage yet).
3. The rep was having a bad day.
4. ... or any or all of the above. ;)

The link I provide you is what Apple has made available to the general public and I believe the rep would have access to both this and an internal knowledge base ... but I'm only guessing as I don't work for Apple.

Any of those or the 18 watt one. See the following from Fast charge your iPhone - Apple Support

Fast charging gives you a quick and convenient way to recharge your iPhone 8 or later. Your iPhone fast charges up to 50 percent in 30 minutes when you use an Apple USB-C to Lightning cable and one of these adapters:

• Apple 18W, 29W, 30W, 61W, or 87W USB-C Power Adapter
• A comparable third-party USB-C power adapter that supports USB Power Delivery (USB-PD)

Yes. I found the 29W to perform this task perfectly. Any additional wattage doesn't appear to make any difference as the Apple chargers are actually smart devices and know when and how to perform "normal" vs. "fast" charging ... or whether a charge is needed at all.

Also, supporting this and taken directly from the Apple support article:

Testing conducted by Apple in August 2017 using preproduction iPhone X units and software and accessory Apple USB-C Power Adapters (29W Model A1540, 61W Model A1718, 87W Model A1719). Fast-charge testing conducted with drained iPhone units. Charge time varies with environmental factors; actual results will vary.

All of them. However, I would suggest going with the 29W model.

Ref: Fast charge your iPhone - Apple Support