The magnetic flux φ linked with a conducting loop varies with time...

According to Faraday's law of electromagnetic induction, the magnitude of the induced emf is given by the rate of change of magnetic flux, which corresponds to the slope of the $\phi-t$ graph.

For the time interval from $t = 0$ to $t = 4\text{ s}$, the graph is a straight line. The slope of this line is constant and is calculated as:
$\text{Slope} = \dfrac{\Delta \phi}{\Delta t} = \dfrac{20 - 0}{4 - 0} = 5\text{ Wb/s}$

Since the slope is constant throughout this interval, the magnitude of the induced emf at $t = 3\text{ s}$ is equal to the magnitude of the slope.

$|e| = 5\text{ V}$

Reading the flux value at $t = 3\text{ s}$ ($15\text{ Wb}$) gives the instantaneous flux, not the emf.

Answer: $5\text{ V}$