D dies January 1 2023 owning a bond worth $3M and a single stock position worth $9M. On July 1, the bond is worth $4M and the stock position is worth $2M. On October 1, the bond is worth $5M and the stock is worth $15M. D's executor files an estate tax return. D has made no lifetime gifts and owns no other assets. What value for the gross estate should be reported on D's estate tax return?

a) $5M

b) $6M

c) $12M

d) $13M

e) $20M