If the greatest value of the term independent of ‘\( x \)’ in the expansion of \( \left(x{\sin \alpha + a}+\dfrac{\cos \alpha}{x}\right)^{10} \) is \( \dfrac{10!}{(5!)^2} \), then the value of ‘\( a \)’ is equal to:
\( -2 \)
\( -1 \)
\( 1 \)
\( 2 \)