Following, are some transactions that have been considered from the standpoint of their estate tax significance upon the death of the transferor. To what extent, if at all, do they constitute transfers subject to gift tax at the time of the transfer?

a.) Near D's death, which, as anticipated, occurred one year later,D gave an insurance policy on D's life to Child C.

b.) D transferred securities to a trust, retaining the right to the income for D's life and providing for payment of the corpus at D's death to unrelated X or X's estate.

c.) D transferred securities to a trust under the terms of which the income was to be paid to unrelated X for X's life. Upon X's death, the corpus was to be returned to D if D was living and, if not, it was to be paid over to unrelated Y or Y's estate.

d.) D transferred securities to a trust under the terms of which the income was to be paid to S for S's life. Upon S's death, the remainder was to go to X or X's estate. However, D retained an unlimited right to alter the terms of the trust in any manner.