What does x denote in 74x151 ic?

Answer 1

It's the "family" the device belongs to.

The 74151 is the original Transistor-Transistor Logic (TTL) device.

The 74L151 is the low-power ("L") version. Slower than the original TTL family.

Others are:

S - Schottky (high-speed)

LS - low-power high speed. About equivalent in speed to the original TTL, but lower power.

ALS advanced low-power high-speed

C - Complementary Metal-Oxide Silicon, which uses a different technology (CMOS rather than bipolar transistors), but conforms to the "74"-series logic functions. Much lower power consumption than TTL devices described above.