Question

how is option "a" the answer?

Answer 1

As we know
A×B=|A|. |B| sin@
where angle b/w A&B is @

&B×A=|A|. |B| sin(π-@)
For equating. LHS=RHS

we need a zero value result of sin or
sin value which has value such that sin@=sin(π-@) ..

here only sinπ. has zero value which say sfy the given condition