Which revision correctly uses an appositive phrase to combine these two sentences?

my brother's prized possession is a small toy ambulance. he often keeps it in his pocket.

a. my brother often keeps a small toy ambulance in his pocket because it is his prized
possession.
b. my brother often keeps a small toy ambulance in his pocket, which is his prized
possession.
c. my brother's prized possession is a small toy ambulance, and he often keeps it in his
pocket.
d. my brother often keeps his prized possession, a small toy ambulance, in his pocket.