The penetration tester is using a password brute force attack, which is a type of password guessing attack that involves trying many possible combinations of passwords against a single username or account. A password brute force attack can be effective when the password is known to be weak, simple, or predictable, such as a default or temporary password. In this case, the penetration tester knows that the help desk analysts change users' passwords to 123change when they request password resets, and decides to brute force the webmail with this password and a list of usernames. A password brute force attack can be done by using tools such as Hydra, which can perform parallelized login attacks against various protocols and services1. The other options are not techniques that the penetration tester is using. SQL injection is a type of attack that exploits a vulnerability in a web application that allows an attacker to execute malicious SQL statements on a database server. Password spraying is a type of password guessing attack that involves trying one or a few common passwords against many usernames or accounts. Kerberoasting is a type of attack that exploits a vulnerability in the Kerberos authentication protocol that allows an attacker to request and crack service tickets for service accounts with weak passwords.

The attacker is sequentially changing the serviceID parameter in the URL, likely in an attempt to access objects that they are not authorized to see. This is indicative of an attempt to exploit an Insecure Direct Object Reference (IDOR) vulnerability, where unauthorized access to objects can occur by manipulating input or changing parameters in the URL.
An insecure direct object reference (IDOR) vulnerability occurs when an application exposes a reference to an internal object, such as a file, directory, database record, or key, without any proper authorization or validation mechanism. This allows an attacker to manipulate the reference and access other objects that they are not authorized to access. In this case, the attacker was trying to exploit the IDOR vulnerability in the servicestatus.php script, which accepts a serviceID parameter that directly references a service object. By changing the value of the serviceID parameter, the attacker could access different services that they were not supposed to see. References: The Official CompTIA PenTest+ Student Guide (Exam PT0-002) eBook, Chapter 4, Section 4.2.2: Insecure Direct Object References; Best PenTest+ certification study resources and training materials, Section 1: Cross-site Scripting (XSS) Attack.