Final answer:
Lidocaine is the spinal anesthetic that can increase its duration when combined with epinephrine.
Step-by-step explanation:
The spinal anesthetic that can increase its duration when combined with epinephrine is b) Lidocaine. Lidocaine is commonly used in combination with epinephrine to prolong the effects of the anesthetic. Epinephrine causes vasoconstriction, which reduces the blood flow in the area of administration and slows down the absorption of lidocaine, leading to a longer duration of action.