Glycolate induces opening of stomata in
(a) Presence of oxygen
(b) Low $C{{O}_{2}}$ concentration
(c) High $C{{O}_{2}}$ concentration
(d) Absence of $C{{O}_{2}}$

Hint: Zelitch in 1963 proposed that the production of glycolic acid in the guard cell is an important factor in stomatal opening. Glycolate production depends upon the amount of gas which is exhaled by humans.

Complete answer:
Under low concentration of $C{{O}_{2}}$ glycolate is produced. Glycolate gives rise to carbohydrate, thus raising the osmotic pressure and also that it could participate in the production of ATP. This might provide the energy required for the opening of stomata.
Low $C{{O}_{2}}$ conc. promotes stomatal opening, while high $C{{O}_{2}}$ conc. inhibits stomatal opening through its acidification of the guard cell thus inhibits PM hyperpolarization. When $C{{O}_{2}}$ level decreases or photosynthesis is needed, guard cells will take up water and swell to open the pores, in order to take more $C{{O}_{2}}$. When $C{{O}_{2}}$ levels increase or the water stress overrides the photosynthesis, the guard cells will close.

Additional Information:
- Theory of Photosynthesis in Guard Cells:
Von Mohl in 1856 observed that stomata open in light and close in the night. He proposed that chloroplasts present in the guard cells photosynthesize in the presence of light resulting in the production of carbohydrate due to which osmotic pressure of guard cells increases.
- Active ${{K}^{+}}$ Transport or Potassium Pump Theory and Role of Abscisic Acid:
The concept of ${{K}^{+}}$ ion transport was given by Fujino. It was supported and elaborated by Levitt & Rashke in 1975 which appears to be an active mechanism which needs ATP.
- Role of Plant Hormones in Stomatal Movements:
CYTOKININ: Presence of Cytokinin (Plant growth regulator) is needed for the active uptake of ${{K}^{+}}$ ions
ABA: Presence of ABA or abscisic acid, a plant growth- inhibiting hormone) favours closing of stomata by blocking uptake of ${{K}^{+}}$ by guard cells in the dark. It also prevents the efflux of ${{H}^{+}}$ ions from guard cells. ABA and $C{{O}_{2}}$ cone, together help in lowering the pH in guard cells and making the medium acidic. This helps in closing the stomata. ABA acts as a stress hormone during drought conditions.
So, the correct answer is, ‘Low $C{{O}_{2}}$ concentration.’

Note:
- Stomata open when the leaf contains enough water. When there is a water shortage, they close. At night, no photosynthesis.
- Stomata close, preventing unnecessary loss of water. Sunny morning photosynthesis is demanding, the supply of water is abundant.
- Stomata of most plants open in the day and close at night, while CAM plants are just the opposite.
- Stomata openings are sensitive to the red light and blue light, and blue light is more effective, it stimulates opening by blue- light receptor zeaxanthin.