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Answer: A. Understanding: We need a subnet mask that simultaneously satisfies: - At least $4$ subnets - At least $50$ usable hosts per subnet Formula: $$\text{Subnets} = 2^n, \quad \text{Usable hosts} = 2^h - 2$$ where $n$ = subnet bits borrowed, $h$ = host bits remaining, and $n + h = 8$ (since we start with a /24). Step 1: Find required subnet bits $$2^n \geq 4 \Rightarrow n \geq 2$$ Step 2: Find required host bits $$2^h - 2 \geq 50 \Rightarrow 2^h \geq 52 \Rightarrow h \geq 6$$ Step 3: Check feasibility with $n + h = 8$ With $n = 2$ and $h = 6$: $2 + 6 = 8$ ✓ $$\text{Subnets} = 2^2 = 4, \quad \text{Usable hosts} = 2^6 - 2 = 62$$ Both conditions are satisfied. Step 4: Calculate the subnet mask Prefix length $= 24 + 2 = 26$. The last octet of the mask has $2$ bits set: $$11000000_2 = 192$$ Subnet mask $= 255.255.255.192$. Verification: With /27 ($n=3, h=5$): usable hosts $= 2^5 - 2 = 30 < 50$ — insufficient. Answer: The most efficient subnet mask is $255.255.255.192$, providing 4 subnets with 62 usable hosts each. $$\boxed{255.255.255.192}$$ Quick Tip: Always verify BOTH constraints — subnets and hosts. A /27 gives more subnets but fails the host requirement, making it a common trap.