Consider a computer system with three users: Alice, Bob, and Cathy. Assume that Alice owns the file afile, and Bob and Cathy can read it. Cathy can read and write Bob's file bfile, but Alice can only read it. Only Cathy can read and write her file cfile. Also assume that the owner of each of these files can execute his/her file. Required:
Create the corresponding access control matrix.

          afile     bfile     cfile

Alice    --x               r--       ---      

Bob              r--             --x       ---

Cathy      r--       rw-           rwx

The above table has - sign which represents the access that is not provided to the user. For example r-- in bfile means that Alice can only read bfile and two dash -- after this show that Alice cannot write and execute bfile.

If you want the access control matrix to show the owner, group and other permission too then you can alter this table as given in Explanation section.

Explanation:

In the table

r = read

w = write

x = execute

There are three users:

Alice

Bob

Cathy

There are 3 files

afile owned by Alice

bfile  owned by Bob

cfile  owned by Cathy

First statement:

Alice owns the file afile, and Bob and Cathy can read it

This means Bob and Cathy has read (r) access to afile

afile -> [(Bob,r) (Cathy,r)]

Second statement:

Cathy can read and write Bob's file bfile:

bfile -> [Cathy, rw)]

This means Cathy has read write (rw) access to bfile

Third statement:

Alice can only read Bob's file bfile

bfile -> [(Alice, r)]

This means Alice has only read (r) access to bfile

Fourth statement:

Only Cathy can read and write her file cfile

cfile -> [(Cathy, rw)]

Fifth statement:

owner of each of these files can execute his/her file

afile -> [(Alice, x)]

bfile -> [(Bob, x)]

cfile -> [(Cathy, x)]

So this makes the matrix:

          afile     bfile     cfile

Alice    --xo             r---            

Bob              r---            --xo      

Cathy      r---       rw--          rwxo

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i guess the correct option is letter a.

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