Final answer:
A longbow is the type of bow that has straight limbs which form an arc when strung, distinguishing it from recurve bows, compound bows, and crossbows.
Step-by-step explanation:
The type of bow with straight limbs that form an arc when strung is known as a longbow. Other options provided such as the recurve bow, compound bow, and crossbow have different designs. The recurve bow has tips that curve away from the archer when the bow is unstrung, the compound bow uses a system of pulleys and cables, and the crossbow has a horizontal limb set on a stock that shoots projectiles.