Let P= Rajeev uses the Internet,
Q= he dreams about spiders.
Whenever Rajeev uses the Internet, he dreams about spiders. which can be represt as $P\rightarrow Q$ Which is equivalent to
$Q'\rightarrow P'$
$\text{OPTION (1): AD}$ Means whenever rajeev does not dream about spiders , rajeev does not use internet which is $Q'\rightarrow P'$ so it is vaild .
$\text{Option(2): DC}$ whenever Rajeev did not use the Internet , Rajeev dreamt about spiders which is $P'\rightarrow Q$
SO it is not vaild
$\text{OPTION (3): CB}$ whenever Rajeev dreamt about spiders, Rajeev used the Internet.which is $Q\rightarrow P$
so it is not vaild .
$\text{OPTION (4): DA}$ whenever Rajeev did not use the Internet.,Rajeev did not dream about spiders.which is $P'\rightarrow Q'$
Its also not vaild .
hence, Answer$:(1)$