Which of the following is not a form of proportional representation?

Plurality system is not a form of proportional representation. In plurality system, the candidate with the highest number of votes in a given constituency is declared the winner, regardless of whether or not the candidate has received an absolute majority of votes. This system does not take into account the proportion of votes received by each party or candidate, and does not ensure that the final composition of the parliament or assembly is proportional to the overall preferences of the voters. In contrast, the other three systems listed - Alternative vote, Second ballot, and List system - are all forms of proportional representation that attempt to ensure a fairer distribution of seats based on the proportion of votes received by each party or candidate.