The comfortable midpoint in one's range is called the resonance tonal expression optimum pitch

The comfortable midpoint in one's range is called the optimum pitch.

Explanation:

Our voices can be used at several different levels. Some will certainly feel more comfortable than others depending on each person. We all have a habitual level, the pitch in which we normally talk. However, the habitual pitch isn't necessarily the best one. The optimum pitch is the level in which it feels the most comfortable to speak, the level in which our voices sound natural. When using the optimum pitch, we do not get tired from speaking, that is, we do not strain our vocal tract.

the answer would be c   i believe thats the most reasonable answer

answer: b) bubbling its way

explanation: an adjective is a word or a group of words that modifies a noun (name, thing, place,

in this case, the first option "to heartily enjoy" isn't describing or modifying any noun, the third option "hiking along merrily" is describing an action but it doesn't modify any noun, the last option "beside us" is just a preposition that indicates position, so the correct answer is the second one: "bubbling its way" because it is modifying the noun "stream".