1. In Young’s double slit experiment, the fringe width is given by

$\beta = \frac{\lambda D}{d}$

where:

• $\lambda$ = wavelength of light
• $D$ = distance between slit and screen
• $d$ = slit separation

2. When the source is changed from red to violet, the wavelength decreases because

$\lambda_{\text{violet}} < \lambda_{\text{red}}$

3. Since fringe width $\beta$ is directly proportional to wavelength,

$\beta \propto \lambda$

so on changing to violet light,

$\beta_{\text{violet}} < \beta_{\text{red}}$

This means the fringes become narrower, i.e. consecutive fringes come closer together.

4. Now check each option:

• A: the fringes will become brighter.
  Brightness does not increase simply because the wavelength changes from red to violet. So this is incorrect.

• B: the intensity of minima will increase.
  In ideal YDSE, minima remain dark due to destructive interference. So this is incorrect.

• C: consecutive fringe lines will come closer.
  Correct, because fringe width decreases with decreasing wavelength.

• D: the central bright fringe will become a dark fringe.
  The central fringe remains bright since path difference there is zero. So this is incorrect.

5. Therefore, the correct option is

$\boxed{\text{C}}$