Define $[x]$ as the greatest integer less than or equal to $x$, for each $x\in \left (- \infty, \infty \right ).$ If $y = [x]$, then area under $y$ for $x\in \left [ 1,4 \right ]$ is _______.

1. $1$
2. $3$
3. $4$
4. $6$

The graph of  $y = [x]$ for $x \in [1, 4]$.

The required area $= (1 \times 1) + (1 \times 2) + (1 \times 3) = 1 + 2 + 3 = 6 \;\text{unit}^{2}. \quad [\because$ Area of rectangle =  Base $\times$ Height $]$

So, the correct answer is $(D).$

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