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Match the following flag bits used in the context of virtual memory management on the left side with the different purposes on the right side of the table below.$$\small \begin{array}{ll|ll} \rlap{\textbf{Name of the bit}} && \rlap{\textbf{Purpose}} \\\hline  \text{I.}  &\text{Dirty} & \text{a.}& \text{Page initialization} \\
\text{II.} & \text{R/W} & \text{b.}& \text{Write-back policy} \\ 
\text{III.} &\text{Reference  } \quad& \text{c.}& \text{Page protection}  \\
 \text{IV.} &\text{Valid} & \text{d.}& \text{Page replacement policy}  \\ \end{array}$$

  1. $\text{I-d, II-a, III-b, IV-c}$ 
  2. $\text{I-b, II-c, III-a, IV-d}$
  3. $\text{I-c, II-d, III-a, IV-b}$
  4. $\text{I-b, II-c, III-d, IV-a}$
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Option (D).

Dirty bit : The dirty bit is set when the processor writes to (modifies) this memory. The bit indicates that its associated block of memory has been modified and has not been saved to storage yet. Dirty bits are used by the CPU cache and in the page replacement algorithms of an operating system.

R/W bit : If the bit is set, the page is read/write. Otherwise when it is not set, the page is read-only.

Reference bit is used in a version of FIFO called second chance (SC) policy, in order to avoid replacement of heavily used page.. It is set to one when a page is used heavily and periodically set to $0$. Since it is used in a version FIFO which is a page replacement policy, this bit is come under category of page replacement.

Valid bit is not used for page replacement. It is not used in any page replacement policy. It tells the page in the memory is valid or not. If it is valid it is directly used and if it is not then a fresh page is loaded. So, basically it is page initialization, because we are not replacing, it is initializing, we not knocking out somebody, we are filling empty space, so initialization, so option (D).

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