The correct answer is option 'D' - A, B and C only.

Let's discuss each option in detail and understand why they are relevant to gas chromatography (GC):

A. Electron Capture:
Electron capture is a detection technique used in gas chromatography. In this technique, a radioactive source emits beta particles, which are captured by the analyte molecules in the gas phase. This leads to the formation of negative ions, which can be detected and measured. Electron capture is particularly useful for the detection of electron-capturing compounds such as halogenated compounds, nitro compounds, and some pesticides.

B. Deemter Equation:
The Deemter equation is an empirical equation that describes the relationship between plate height (H) and linear flow velocity (u) in a chromatographic column. The equation is given as H = A + Bu + Cu^2, where A, B, and C are constants. The Deemter equation is used to optimize the separation efficiency of a chromatographic column by balancing the contributions of different factors such as longitudinal diffusion, resistance to mass transfer, and eddy diffusion.

C. Partition Coefficient:
Partition coefficient, also known as distribution coefficient, is a measure of the distribution of a solute between two immiscible phases, such as a stationary phase and a mobile phase in gas chromatography. It is defined as the ratio of the concentration of the solute in the stationary phase to the concentration of the solute in the mobile phase at equilibrium. The partition coefficient determines the degree of retention of a solute in the stationary phase, which affects its separation and elution time in gas chromatography.

D. Critical Temperature of Gas:
The critical temperature of a gas is the temperature above which the gas cannot be liquefied, regardless of the pressure. It is an important property to consider in gas chromatography because it determines the operating conditions for the separation. The critical temperature of a gas influences its behavior in the chromatographic column and affects factors such as diffusion, mass transfer, and retention time.

In conclusion, all the options A, B, and C - Electron Capture, Deemter Equation, and Partition Coefficient - are relevant to gas chromatography (GC). The critical temperature of a gas (option D) is also relevant as it affects the chromatographic separation. Therefore, the correct answer is option 'D' - A, B, and C only.