How the words from a language other than English are written in a research paper?

The correct option is In Italics.

In formal academic writing, including research papers, standard style guides (such as APA, MLA, and Chicago Manual of Style) mandate that words or phrases from a language other than English be formatted in italics.
This styling convention is used for several reasons:
1. Visual Distinction: It immediately alerts the reader that the word belongs to another language, signaling a potential shift in pronunciation or cultural context.
2. Prevention of Confusion: It helps differentiate foreign terms from English words that might look similar but have entirely different meanings.
3. Standardized Citation: It is a universally accepted academic format for integrating non-anglicized terms (such as weltanschauung or force majeure) smoothly into English prose.

In contrast, formatting styles like bold letters, underlined text, or capital letters are reserved for other structural purposes in a manuscript, such as headings, strong emphasis, or acronyms, and are not used to denote foreign vocabulary.