Most housing within the various ethnic enclaves could be considered what?

The right answer is Tenements

Explanation:

During the 19th century with the rising of immigrants in the US, buildings whose previous owners were single-family were divided into various living spaces to adapt to the population. There were narrow, low-rise apartments that were overcrowded, unwell set alight and were deficient in indoor sanitation and suitable ventilation. By 1900 more than two-thirds of the population were living in this kind of building known as tenement.

It would be considered Tenements.