## 1 Introduction  
Wireless sensor networks (WSNs) have become a crucial technology in the proliferation of Internet of Things (IoT) use cases and application scenarios across a wide range of fields. These WSN-based IoT applications include smart industrial monitoring [1], smart transportation [2], real-time tracking and mapping [3], smart agricultural irrigation [4], smart grids [5], smart homes and smart buildings [6], smart cities [2,7], smart exploration [8], and military reconnaissance [9], as well as applications related to safety, stability maintenance, surveying, medical, and health [10]. WSN are considered a key enabling technology for IoT due to their numerous applications. A WSN can be described as a large number of small size, low-cost, battery-operated sensor nodes (SNs) that work collaboratively in the field of interest (FoI) to monitor environmental and physical conditions, such as temperature, noise, smog, humidity, pressure, chemical level, light, fire detection, etc. [3,9,11]. Each SN in a WSN is able to sense, process, and transmit data to other SNs or a base station (BS) either directly or upon request. SNs are typically battery-limited and deployed in remote, inaccessible, hazardous, and dynamic environments [3,11,12], making it practically impossible to replace or recharge their batteries. Various factors such as the inaccessibility to the FoI, limited energy resources, and high cost of maintenance contribute to this challenge. Consequently, energy-efficient and effective routing protocols and techniques are necessary to prolong the lifetime of SNs without requiring battery replacement or recharging. Routing protocols for WSNs are broadly classified into two types: (i) flat and (ii) hierarchical routing protocols [13,14]. In flat routing protocols, all the SNs in WSN play the same role. The packets flow in multihop manner, i.e., the packet is sent to destination or BS through multiple SNs (hops) which are in close proximity to each other; thus, the transmission distance is minimized between the particular SN and the BS, and the energy is saved. Some of the flat routing protocols which are proposed for WSNs are rumor routing (RR) [15], wireless routing protocol (WRP) [16], flooding [17], etc. In flat routing protocols, an SN forwards its data packets to the next SN in the path to the BS. As a result, SNs near to the BS receive and forward data packets repeatedly from multiple SNs due to their physical locations, and they consume more radio energy than the other SNs in the FoI. Thus, these frequently used SNs die early and the problem known as 'SN hotspot' occurs in flat routing protocols. To solve this problem, hierarchical routing protocols were proposed. In hierarchical routing protocols, all deployed SNs are divided into groups called clusters, and each cluster selects one of its SNs to serve as the cluster-head (CH), which collects data packets from all its cluster members (CMs). The CMs only sense the surrounding and share the sensed data with the particular CH. The CH consumes more energy and dies early as compared to CMs, because of the reception of data packets from all its cluster member SNs, data aggregation, and forwarding all the collected data packets to the BS. Hence, efficient CH selection from SNs in WSN is a challenging task in hierarchical routing protocols. Such protocols have some benefits over the flat routing protocols, including scalability, load balancing, reducing routing delays, and energy-efficiency. Eminent cluster-based protocols include Power-Efficient Gathering in Sensor Information Systems (PEGASIS) [18], Low Energy Adoptive Clustering Hierarchy (LEACH) [19], Low Energy Adaptive Clustering Hierarchy Centralized (LEACH-C) [20], Stable Energy Protocol (SEP) [21], Threshold sensitive energy Effective Sensor Network protocol (TEEN) [22], Improved Energy Efficient LEACH (IEE-LEACH) [23], and Enhanced Energy Efficient Clustering Approach for Three-tier Heterogeneous Wireless Sensor Networks (EEECA-THWSN) [24].