The class decided to organise __________.

The correct answer is 'a picnic'. Since 'picnic' is a singular countable noun being introduced for the first time, the indefinite article 'a' is required. 'Picnic' without an article is ungrammatical, 'picnics' (plural) does not fit the singular intent, and 'the picnic' would imply a specific, previously mentioned picnic.