• Non-Probability Sampling: When individual sampling units in a target population do not have a known or equal non-zero probability of being selected, the procedure is termed non-probability sampling, the foremost example being convenience sampling (accidental/haphazard sampling).
• Probability Sampling Methods (where every unit has a defined non-zero probability of selection):
• Simple Random Sampling (SRS): Every unit has an identical, equal probability of selection ($p = 1/N$).
• Stratified Random Sampling: Population divided into homogeneous strata, with random samples drawn from each stratum.
• Cluster Sampling: Heterogeneous naturally occurring primary clusters (e.g., villages, herds) are randomly sampled.

Correct option: (4) Convenience sampling