Final answer:
The slogan "Life is better with the Conservatives, don't let Labour ruin it" was from the successful Conservative general election campaign of 1959, when Prime Minister Harold Macmillan led the party to win a third successive term.
Step-by-step explanation:
The slogan "Life is better with the Conservatives, don't let Labour ruin it" was used in a successful general election campaign by the Conservative Party, which corresponds to option C) 1959. During the election of 1959, the Conservative Party was led by Prime Minister Harold Macmillan, who campaigned on the economic prosperity of the period. The Conservatives won a third successive election, capitalizing on the nation's sense of well-being and the slogan helped to rally voters to support the continuance of Conservative governance, emphasizing the risk of Labour's policies potentially undermining the country's prosperity.