Since the polynomial has the highest degree $7$. So there are $7$ roots possible for it
Now suppose if an imaginary number $a+bi$ is also the root of this polynomial then $a-bi$ will also be the root of this polynomial
That means there must be an even number of complex roots possible because they occur in pairs.
Now we will solve this question option wise
A) All complex root
This is not possible. The polynomial has $7$ roots and as I mention a polynomial should have an even number of complex roots and $7$ is not even. So this option is wrong
B) At least one real root
This is possible. Since polynomial has $7$ roots and only an even number of the complex root is possible, that means this polynomial has max $6$ complex roots and Hence a minimum of one real root. So this option is correct
C) Four pairs of imaginary roots
$4$ pair means $8$ complex root. But this polynomial can have at most $7$ roots. So this option is also wrong
Hence answer should be (B).