The correct answer is C. Is.

Explanation:

1. Is: This option is correct because "three kilometers" is treated as a singular concept in this context, referring to a single measurement of distance. In English, distances and quantities often take a singular verb when considered as a whole unit.

2. Are: This option is incorrect because "are" is a plural verb form. Although "kilometers" is plural, the phrase "three kilometers" is treated as a singular measurement, so the singular verb "is" is appropriate.

3. Have been: This option is incorrect because "have been" is a plural form and is used with plural subjects or in a different tense structure, which does not fit the context here.

4. Were: This option is incorrect because "were" is the past tense plural form and does not fit the present tense context of the sentence.

Summary:

- C. Is is the correct choice because it correctly matches the singular concept of "three kilometers" when considered as a single unit of distance.