The non-trivial $\text{FD}$s are
- (sname, city) $\to$ street
- sid $\to$ street
- (sname, city) $\to$ sid
- sid $\to$ sname
- sid $\to$ city
For all these, $\text{LHS}$ is a super key and hence $\text{BCNF}$ condition is satisfied. But we have some more dependencies here:
"each supplier and each street within a city has unique name"
This basically means each supplier in a city has unique name making (sname, city) determine sid and hence making it a candidate key. Each street within a city also has a unique name and so (street, city) is also a candidate key. Even then with all $3$ candidate keys (for Suppliers schema), for any $\text{FD}$, the $\text{LHS}$ is a super key here, and hence the relation schema (for other two relations it is straight forward) is in $\text{BCNF}$.
http://db.grussell.org/section009.html
Correct Answer: $A$