Given equation

$y = 2a \sin\left(\frac{2\pi nt}{\lambda}\right) \cos\left(\frac{2\pi x}{\lambda}\right)$

Use the rule for arguments of trigonometric functions

The arguments of $\sin$ and $\cos$ must be dimensionless.

So, $\frac{2\pi nt}{\lambda} \text{ is dimensionless}$ and $\frac{2\pi x}{\lambda} \text{ is dimensionless}$

Since $2\pi$ is dimensionless, we get: $\frac{nt}{\lambda} \text{ is dimensionless}$ and $\frac{x}{\lambda} \text{ is dimensionless}$

From $\dfrac{x}{\lambda}$ dimensionless

$[x] = [\lambda] = [L]$

So option C is correct.

From $\dfrac{nt}{\lambda}$ dimensionless

$[nt] = [\lambda] = [L]$

So option A is correct.

Find dimensions of $n$

Since $[nt] = [L]$ and $[t] = [T]$, $[n] = \frac{[L]}{[T]} = [LT^{-1}]$

So option B is correct.

Find dimensions of $\dfrac{n}{\lambda}$

$\left[\frac{n}{\lambda}\right] = \frac{[LT^{-1}]}{[L]} = [T^{-1}]$

But option D says the dimensions of $\dfrac{n}{\lambda}$ is $[T]$, which is wrong.

Conclusion

The statement which is NOT correct is: $\boxed{\text{D}}$