We are given,

The mean and standard deviation of Mathematics are $42$and $12$ respectively.

The mean and standard deviation of Physics are $32$ and $15$ respectively.

The mean and standard deviation of Chemistry are $40.9$ and $20$respectively.

Also, we know that coefficient of variation is,

\[C.V = \frac{{{\text{ Standard deviation }}}}{{{\text{ Mean }}}} \times 100\]

Now,

C.V in Mathematics \[ = \frac{{12}}{{42}} \times 100\]

\[ = 28.57{\text{ }}\]

C.V in Physics \[ = \frac{{15}}{{32}} \times 100\]

\[ = 46.87\]

C.V in Chemistry \[ = \frac{{20}}{{40.9}} \times 100\]

\[ = 48.89\]

Therefore, out of the three subjects the highest variability in marks is in Chemistry and the highest variability in marks is in Mathematics.