$R_1 \cap R_2 \neq \phi.$ This makes the decomposition lossless join, as all the attributes are keys, $R_1 \cap R_2$ will be a key of the decomposed relations (lossless condition says the common attribute must be a key in at least one of the decomposed relation). Now, even the original relation $R$ is in $3NF$ (even $\text{BCNF}$)as all the attributes are prime attributes (in fact each attribute is a candidate key). Hence, any decomposition will also be in $3NF$ (even $\text{BCNF}$). Option $D$.
PS: Decomposition in $3NF$ means decomposed relations are in $3NF$. But when we consider any decomposed relation, we must also include any FD which are being implied by the original relational schema. For example, in a decomposed relation $STU,$ there will be a FD $U\to S$ as well.