Final answer:
A grand piano is a necessity for Jenna, a concert pianist, as it is crucial for practicing, developing skills, and performing in concerts. Other options like reward, hindrance, or distraction do not fit in the context of a requirement for her profession. So, the correct answer is option 4.
Step-by-step explanation:
Jenna is a concert pianist, and the correct answer to what a grand piano is to her labor in the production of her concerts is a necessity. A concert pianist cannot perform without their instrument, hence the piano is essential for her to practice, develop her skills, and perform.
Options such as reward, hindrance, and distraction do not accurately fit the context of something that is fundamentally required for a concert pianist to carry out their work.