Select the INCORRECTLY spelt word.

The correct option is "occassion".

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

Therefore, the incorrectly spelt word is "occassion".