Alex should wear the cotton shirt and not the polyester one because:
(i) Cotton is a natural fibre and polyester is a synthetic fibre.
Cotton fibre is safe to wear near fires as it doesn't catch fire easily. Even if it catches fire, it burns slowly.
(ii) On the other hand, clothes made up of polyester fibres catches fire easily and burns rapidly. It melts and sticks to the body of the person wearing it. Thus, it is not advisable to wear synthetic clothes near fire.