Which of these is used for both entering and exiting a highway?

Answer: C — Weave lane.

The weave lane is specifically designed for both entering and exiting a highway. This unique lane allows vehicles to merge onto the highway while simultaneously accommodating vehicles exiting, thus creating a shared space where drivers can transition between the two actions. The design of a weave lane helps to streamline traffic flow by minimizing the need for separate entry and exit ramps, allowing drivers to change their paths more efficiently. This can be particularly useful in high-traffic areas where space is limited. Contrasting with the other options, acceleration and deceleration lanes are designed solely for getting onto or off the highway, respectively. An acceleration lane provides drivers with the space needed to increase their speed to match highway traffic when entering, while a deceleration lane is intended for vehicles to safely reduce speed when preparing to exit. A merge lane, while related to entering traffic, does not serve the dual purpose of facilitating both entry and exit like a weave lane does.