Presence of a non-condensing gas in a condensing vapor does not have any direct effect on the rate of condensation, thermal resistance, or the film coefficient. Therefore, the correct answer is option 'D' - none of these.

Here is an explanation of each option:

a) Increases the rate of condensation:
The presence of a non-condensing gas does not increase the rate of condensation because the non-condensing gas does not participate in the condensation process. Only the condensable vapor present in the mixture will undergo condensation.

b) Decreases thermal resistance:
Thermal resistance refers to the resistance encountered by heat transfer across a system. The presence of a non-condensing gas does not have a direct effect on thermal resistance because the non-condensing gas does not participate in the heat transfer process. It is the condensing vapor that transfers heat during condensation.

c) Is desirable to increase the film coefficient:
The film coefficient is a measure of the effectiveness of heat transfer between a surface and a fluid. The presence of a non-condensing gas does not increase the film coefficient because the non-condensing gas does not participate in the heat transfer process. The film coefficient depends on factors such as fluid properties, flow conditions, and surface characteristics.

Non-condensing gases can, however, indirectly affect the condensation process by diluting the condensable vapor and reducing its concentration. This can potentially decrease the rate of condensation. Additionally, the presence of non-condensing gases can affect the flow behavior and distribution of the condensing vapor, leading to non-uniform condensation. However, these effects are not directly related to the options given in the question.

In conclusion, the presence of a non-condensing gas in a condensing vapor does not have any direct effect on the rate of condensation, thermal resistance, or the film coefficient. Therefore, option 'D' - none of these is the correct answer.