Final answer:
The true statement about bilinguals is that they may naturally mix languages in conversation with other bilinguals, a practice known as code-switching, reflecting their linguistic competence and strategic language use.
Step-by-step explanation:
The correct answer to the question 'Which of the following is true of bilinguals?' is D. A bilingual speaker is likely to mix the two languages in a conversation as natural behavior for bilinguals conversing with other bilinguals. This mixing of languages, known as code-switching, is a normal part of being bilingual and can occur even when speakers are competent in both languages. It is not necessarily indicative of a lack of competence, but rather a sophisticated linguistic skill that allows bilingual individuals to navigate different linguistic and cultural norms. That being said, bilinguals are strategic in their use of language and may opt not to code-switch when conversing with someone who only speaks one language.