A common technique used in transposition ciphers is the "rail fence" method. What does this involve?

Select one:

a$.$ Creating a keyword to guide the letter rearrangements.

b$.$ Substituting vowels with consonants, and vice versa.

c$.$ Dividing the message into equal$-$sized blocks and rearranging them.

d$.$ Write the message in alternating diagonals across lines.