Explanation:

- Size of Group: Larger groups often face challenges in maintaining integration due to diverse perspectives and interests.

- Common History: A shared history can foster unity and understanding within a group, but it's not a primary factor of integration according to Ogburn and Nimkoff.

- Homogeneity: Similar values, norms, and characteristics contribute to social integration by reducing potential conflicts.

- Geographical Mobility: Frequent movement can disrupt established social bonds, affecting integration negatively.

The correct answer is Option:2, common history.

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