What height did the plants in the F1 generation display/have? All tall
50% Tall, 50% Short
75% Tall, 25% Short
All short

Note about the question:

I looked for the complete question but failed in finding the parental genotypes. So in my answer and explanation, I will include different scenarios of different crosses that might result in the given genotype options. You just need to look at the cross that corresponds to the one in your problem. I hope this helps.  

Explanation:

Parental)   TT    x    tt

Gametes) T   T       t   t

Punnett Square)  T       T

                    t      Tt     Tt

                    t      Tt     Tt

F1) 100% Tall, Tt

Parental)   TT    x    TT

Gametes) T   T       T   T

Punnett Square)  T       T

                    T     TT     TT

                    T      TT     TT

F1) 100% Tall, TT

Parental)   Tt    x    tt

Gametes) T   t      t   t

Punnett Square)  T       t

                      t     Tt      tt

                      t     Tt      tt

F1) 50% Tall, Tt

     50% short, tt

Parental)   Tt    x   Tt

Gametes) T   t      T   t

Punnett Square)    T       t

                      T     TT      Tt

                       t      tT      tt

F1) 75% Tall, (25% TT + 25% Tt + 25% tT)

     25% short, tt

Parental)   tt    x    tt

Gametes) t   t      t   t

Punnett Square)    t       t

                       t     tt      tt

                       t     tt      tt

F1) 100%  short, tt

i am going to guess that it is co2.