The correct answer is option 'B' - Socialism.

Socialism gained widespread attention and became an influential idea for restructuring society in Europe by the mid-19th century. This period was marked by significant social and economic transformations fueled by industrialization and urbanization. The emergence of the working class and the harsh working conditions they faced led to a growing discontent with the existing capitalist system.

Below are the reasons why socialism became an attractive idea during this time:

1. Rise of the Industrial Working Class:
- Industrialization led to the growth of factories and the concentration of workers in urban areas.
- The working class faced long working hours, low wages, and poor living conditions.
- This exploitation and inequality created a sense of solidarity among the working class and fueled support for socialist ideas.

2. Criticism of Capitalism:
- Capitalism, characterized by private ownership of the means of production and the pursuit of profit, was seen as the root cause of social inequality and exploitation.
- Socialists argued that capitalism created an unfair distribution of wealth, with a small capitalist class accumulating wealth at the expense of the working class.

3. Social Justice and Equality:
- Socialism advocated for a more equitable distribution of wealth and resources.
- It emphasized the need for social justice, equal opportunities, and access to basic necessities for all members of society.
- Socialists believed that through collective ownership and control of resources, society could be organized in a way that addressed the needs of all individuals.

4. Utopian and Scientific Socialism:
- Utopian socialists, such as Robert Owen and Charles Fourier, proposed idealistic visions of cooperative communities where wealth and resources were shared.
- The ideas of scientific socialism, popularized by Karl Marx and Friedrich Engels in their seminal work, "The Communist Manifesto," provided a more theoretical and analytical framework for socialist thought. They argued for the revolutionary overthrow of capitalism by the working class.

5. Influence of Socialist Movements:
- Socialist movements, including trade unions and political parties, gained momentum during this period.
- The formation of international organizations like the International Workingmen's Association (First International) in 1864 further spread socialist ideas and encouraged cooperation among socialist groups.

In conclusion, by the mid-19th century, socialism attracted widespread attention in Europe due to the rise of the industrial working class, criticism of capitalism, the pursuit of social justice and equality, the influence of utopian and scientific socialism, and the growth of socialist movements.