Answers under questions:

which arrow represents the refracted arrow of incident ray a?
⇒ b

which arrow represents the refracted arrow of incident ray d?
⇒ e

Explanation:

Refraction is a phenomenon in which a wave (the light in this case) bends or changes its direction when passing through a medium with a refractive index different from the other medium.  Where the Refractive index is a number that describes how fast light propagates through a medium or material.  

According to this, if we observe the rays  A an D passing throgh the biconcave lens, we will have two mediums:

1) The air

2)The material of the biconcave lens

This two mediums have different refractive indexes, hence the rays will change the direction.

-For the incident ray A, the corresponding refractive ray is B, because is the ray that bends after passing throgh the lens

-For the incident ray D, the refracted ray is E following the same principle.

d

explanation:

v=uf/(u-f)

= 5.10/(5-10) convex lens has positive focal length

=50/-5

=-10 cm (formed at other side)