In a discharge tube ionization of enclosed gas is produced due to collisions between

(a)Negative electrons and neutral atoms / molecules

(b)Photons and neutral atoms /molecules

(c)Neutral gas atoms/molecules

(d)Positive ions and neutral atoms/molecules

        (a)

When electron emitted from cathode collide with gas molecules or atoms, they knock out outer electrons and produce positively charged ions. They become part of positive rays.

(a)

When electron emitted from cathode collide with gas molecules or atoms, they knock out outer electrons and produce positively charged ions. They become part of positive rays.