I believe the answer is C
I took my dog for a walk is a A) simple sentence
It has only one clause (which is independent) therefore it can't be compound, complex, or compound-complex, since all of these kinds of sentences have more then one clause
Hope this helped!
~Just a girl in love with Shawn Mendes
While really all of these options are true to some extent, "<span>advertisements can be more cost effective" would be the best since companies can target specific people whose likes are directly in line with the product they want to sell. </span>