To determine which word has a different main stress pattern, let's analyze the options:

1. **facility** – stress on the second syllable (fa-CIL-i-ty)
2. **amenity** – stress on the second syllable (a-MEN-i-ty)
3. **exhaust** – stress on the second syllable (ex-HAUST)
4. **affordable** – stress on the second syllable (af-FORD-a-ble)

For the second group:

1. **expensive** – stress on the second syllable (ex-PENS-ive)
2. **convenient** – stress on the second syllable (con-VEN-ient)
3. **congestion** – stress on the second syllable (con-GES-tion)
4. **difference** – stress on the first syllable (DIFF-er-ence)

For the third group:

1. **resident** – stress on the first syllable (RES-i-dent)
2. **downtown** – stress on the first syllable (DOWNTOWN)
3. **variety** – stress on the second syllable (va-RI-e-ty)
4. **concrete** – stress on the first syllable (CON-crete)

For the fourth group:

1. **hospitable** – stress on the second syllable (hos-PIT-a-ble)

The words that have a different stress pattern are:

- From the first group: **exhaust**
- From the second group: **difference**
- From the third group: **variety**
- From the fourth group: All stress patterns vary accordingly.

Thus, the one that stands out with a different main stress pattern compared to the others is:

**D. difference** (it has the stress on the first syllable while others have it primarily on the second.)