The flux density is ___ when the magnetic flux is 9 µWb and the cross-sectional area of the magnetic field is 5 × 10^–3 m².

ANS:A - 1800 µT

It's formula based.

The Magnetic Flux "Density" is denoted as;
Beta(β)[unit T(tesla)].
and Magnetic Flux is denoted as;
Phi(φ)[unit Wb(weber) ].

The Formula β=φ/A.
(Flux Density equals Flux all over Area.)

Thus:
β = (9x10^-6)/(5x10^-3).
β = 1.8mT or 1800μT.