Select the INCORRECTLY spelt word.

The correct answer is occassion.

Let us analyze the spelling of each of the given options to identify the incorrectly spelt word:
1. parallel: This word is spelt correctly. It refers to lines or surfaces that are side by side and having the same distance continuously between them.
2. occassion: This word is spelt incorrectly. The correct spelling is occasion, which contains a double 'c' but only a single 's'. An occasion refers to a particular event, occurrence, or time.
3. sincere: This word is spelt correctly. It means free from pretense or deceit; genuine.
4. triangle: This word is spelt correctly. It refers to a plane figure with three straight sides and three angles.

Therefore, the incorrectly spelt word among the choices is occassion.