The 74138 is a 3-to-8 decoder. Together, two of these ICs can be used to form one 4-to-16 decoder. To do this, connect:
a) Pins 2 and 3 of each IC
b) Pins 1 and 2 of each IC
c) Pins 5 and 6 of each IC
d) Pins 7 and 8 of each IC