REQUIRED for a digital signal to travel from the input(s) of a logic gate to the output ad is DENOTED by (TPD)In the case of a digital IC, the propagation delay of a gate is the AVERAGE transition time that a signal takes from input to output, i.e.\({t_{pd}} = \frac{{{t_{PHL}} + {t_{PLH}}}}{2}\)Units – ns (nanoseconds) i.e. 10-9 secSpeed of OPERATION is related to the propagation delay, so, it is advantages to have smaller tpd’sFamilytpd (ns)MOS70TTL10RTL12ECL2∴ The decreasing order is a, c, b, d