Yes sir, you can wear cotton t-shirt for NEET 2019. Just make sure that it's not high in florals and number of pockets. Moreover, it must be half-sleeved.

All the best for your exam!

Yes you can there's no particular rule for the fabric of your clothes. But make sure the t-shirts should be of light color and half sleeved without any metal buttons or heavy embroidery or designs 

Thank you.

Hope it helps.