Answer:
'tri'
Explanation:
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The apostrophe before the following syllable indicates that the previous syllable is stressed.
From LawlessEnglish.com  "Syllable Stress
"
English words that are polysyllabic (have more than one syllable) always have one syllable that is stressed. In most English dictionaries, the stressed syllable is indicated by a stress mark, a symbol that resembles an apostrophe. The stress mark follows the syllable that is stressed.
For example, in the word incredible, the second syllable (-cred-) is stressed. Here are some examples.
2	3	4
ex′it	op′po·site	Feb′ru·ar·y
hu′mid	ex·am′ine	in·cred′i·ble
ma·chine′	em·ploy·ee′	psy·cho′lo·gy
Here is the Dictionary.com  for retribution retribution noun
ret·ri·bu·tion | \ ˌre-trə-ˈbyü-shən  \
The apostrophe is before the 'byu  so the PREVIOUS syllable is the one that is stressed.