The question asks about the use of "et al." in a research article context. The term "et al." is short for the Latin phrase "et alii," meaning "and others." It is used when citing a source with multiple authors to save space or avoid redundancy. Let's explore the options:

1. Option A (1 Author): If there is only one author, there would not be any need to use "et al." as "et al." implies that there are additional authors.
2. Option B (2 Authors): Typically, for two authors, both names are mentioned, such as "Smith and Jones." Therefore, "et al." would not be used.
3. Option C (3 Authors): In many citation styles, if there are three authors or more, it is permissible to cite the first author followed by "et al." to indicate additional authors. For example, "Smith et al." could represent Smith, Johnson, and Brown.
4. Option D (4 Authors): Similarly, with four authors, it is common to use "et al." after the first author’s name, such as "Smith et al." to imply Smith, Johnson, Brown, and Taylor.

Therefore, the correct options are C (3 Authors) and D (4 Authors). These align with the common practice where "et al." is used to indicate multiple authors beyond the primary one.

Hence, the correct answer is: C and D Only.