New crust are formed at divergent boundary.
Let's look into the options one by one:
a. subduction zone
-Subduction zone is where plates sink into the magma and melt, crust materials are destroyed an therefore is incorrect.
b. divergent boundary
-This is correct.Divergent boundary is where plates move apart given the rising convection current of magma. As cracks are formed due the the slab pull, magma fills the cracks and cools down to form new crust materials.
c. convergent boundary
-This is incorrect.Convergent boundary is where subduction zone can be found and destroys plate material.
d. transform boundary
-This is incorrect. Transform boundary is where plates slide past each other and no material is destroyed or produced.
Therefore the answer is b. divergent boundary.
Hope it helps!