The correct answer is option 4, posterior part. The area of the human tongue that is sensitive to bitterness is mainly located in the posterior part, also known as the back of the tongue. This area contains taste buds that are specifically sensitive to bitter tastes. It is important to note that while the back of the tongue is most sensitive to bitterness, it can also detect other taste sensations, such as sweetness, sourness, and saltiness, albeit to a lesser extent. This sensitivity to different tastes is due to the distribution of taste buds on different parts of the tongue. Therefore, if you want to specifically taste bitterness, it is more likely to be detected at the back of your tongue.