b
Speed of sound in the tissue, $v=1.7 \,km / s =1.7 \times 10^{3} \,m / s$
Operating frequency of the scanner, $v=4.2\, MHz =4.2 \times 10^{6} \,Hz$
The wavelength of sound in the tissue is given as
$\lambda=\frac{v}{v}$
$=\frac{1.7 \times 10^{3}}{4.2 \times 10^{6}}=4.1 \times 10^{-4} \;m$