The correct answer is D. Purposive sampling.
Purposive sampling is a type of non-probability sampling in which the researcher selects the sample based on their judgment about who would be the most informative to include. In this case, the reporter is using purposive sampling to conclude that Congress will win the election because they have seen a very big rally. The reporter is assuming that the people who attended the rally are representative of the general population and that their support for Congress is indicative of how the general population will vote.
Random sampling is a type of probability sampling in which each member of the population has an equal chance of being selected for the sample. Cluster sampling is a type of probability sampling in which the population is divided into clusters and then a random sample of clusters is selected. Systematic sampling is a type of probability sampling in which every nth member of the population is selected for the sample.
In this case, the reporter is not using random sampling, cluster sampling, or systematic sampling because they are not selecting the sample randomly. The reporter is using purposive sampling because they are selecting the sample based on their judgment about who would be the most informative to include.