Answer
$(a)$ a one-turn square coil
Work Step by Step
Induced emf is equal to the rate of change of magnetic flux and magnetic flux is directly proportional to magnetic field.
Write the expression for the induced emf.
$V=NBA\omega sin(\omega t)$
$V \propto NA$
Here, $V$ is the induced emf and $N$ is the number of turns. Since, the area is inversely proportional to the number of turns, as the number of turns increases, the area decreases.
Hence, the correct option is $(a)$
