Step 1: Understanding the Concept:
On a frictionless surface the total mechanical energy is conserved. All the kinetic energy of the block goes into the spring at maximum compression \(x\).
Step 2: Energy balance:
\[ \frac12mv^2 = \frac12Kx^2 \Rightarrow v = x\sqrt{\frac Km} \]
Step 3: Momentum:
\[ p = mv = m\,x\sqrt{\frac Km} = x\sqrt{Km} \]
This is the largest momentum the block carries, since its speed only falls once the spring is compressed.
Step 4: Check the options:
Option (B) \(\sqrt{km}\,x\) matches. Option (A) is the momentum at the instant of maximum compression. Options (C) and (D) have units that do not match momentum: \(mx^2/K\) and \(Kx^2/2m\) are not kg m/s.
Final Answer:
Momentum is m times x times root of K over m.
\[ \boxed{\text{(B) }\sqrt{Km}\,x} \]