The correct answer is (c) been.

Explanation:
The sentence "The environment has always been in control of our density" is written in the present perfect tense. In English grammar, the present perfect tense is used to describe an action or state that started in the past and continues up to the present. It is formed using the auxiliary verb "has" or "have" followed by the past participle of the main verb.

In this sentence, the word "always" indicates that the control of our density by the environment has been a continuous and ongoing process. The verb "been" is the past participle form of the verb "be." It is used to indicate the existence or occurrence of something in the past or present.

Here is a detailed breakdown of the sentence:

1. The environment: This refers to the natural surroundings or conditions in which living organisms exist.

2. has always been: The phrase "has always been" indicates the present perfect tense. "Has" is the auxiliary verb, and "been" is the past participle of the verb "be."

3. in control of: This phrase means that the environment has the power or authority to regulate or determine something. In this case, it refers to the control of our density.

4. our density: This refers to the concentration or number of individuals within a specific area or space.

By using the present perfect tense with the word "always," the sentence suggests that the environment has continuously and consistently had control over our density. It implies that the environment has influenced or shaped the population density of humans or other organisms over time.

Been...