a
    According to https://en.wikipedia.org/wiki/Control_register#CR4, the SMAP and SMEP bits are used to protect ring 0.
    If writing to CR4 would be allowed, we could disable this protection, and access kernel space data (e.g.)

b
    0x40050e <main+8>           mov     %cr4,%rax

    This attempts to move the RAX register to the CR4 register, i.e. to write to CR4.

c
    Done. I suppose it isn't necessary to give you the code or results, as all was needed was copying from the given website.

d
    See hello-cr4.c and Makefile.
    
    # make
    [..]
    # insmod hello-cr4.ko
    [15892.352286] Hello world! CR4 =         7f0