Question

to expand their empires, imperialist countries in the 1800s preferred to take over
other imperialist countries with experienced labor forces.
industrially undeveloped countries rich in natural resources.
other imperialist countries that needed raw materials.
industrially undeveloped countries that needed resources.

Explanation:

Imperialist countries in the 1800s aimed to expand their empires. They were interested in resources for their industries. Industrially - undeveloped countries rich in natural resources would provide the raw materials (like minerals, agricultural products) that the imperialist countries' industries needed. Taking over other imperialist countries (which were also industrializing and might not have abundant untapped resources) or countries that "needed resources" (as they likely couldn't offer the resources imperialists wanted) was less beneficial.

Answer:

"industrially undeveloped countries rich in natural resources"