Question

To concatenate s1, s2, s3, and s4, you write ________.

Answer 1

I would say that since concatenate is to link things together in a chain or series I would write this series as s1+s2+s3+s4. In other words, the + sign would indicate the links between the individual items which together would make a chain.

Answer 2

According to Standard C, the statement
s1 = strcat(s1, s2) will concatenate string s2 to s1. That is, s2 will be appended to the end of  s1.

Therefore, to concatenate s1, s2, s3, and s4, write
s1 = strcat(s1, (strcat(s2, strcat(s3, s4))))
and the result will be stored in s1.