Consider a CSMA/CD network that transmits data at a rate of 100 Mbps ($10^8$ bits per second) over a 1 km (kilometer) cable with no repeaters. If the minimum frame size required for this network is 1250 bytes, what is the signal speed (km/sec) in the cable?

In a CSMA/CD network, to ensure collision detection before transmission ends, the transmission time must be at least twice the propagation delay [12]:

Substituting the formulas for transmission time $T_t = \frac{L}{B}$ and propagation delay $T_p = \frac{d}{v}$:

Given data:

• Bandwidth ($B$) = $100\text{ Mbps} = 10^8\text{ bits/second}$

• Distance ($d$) = $1\text{ km}$

• Minimum frame size ($L$) = $1250\text{ bytes} = 1250 \times 8\text{ bits} = 10^4\text{ bits}$

Substitute the values to find the signal speed ($v$):

Hence, option (D) is correct.