Validation controls are used to check the input data from the user before processing it on the server. If the validation controls are moved from the server side to the browser, it means that the user can modify or bypass them using tools such as browser developer tools, JavaScript console, or proxy tools. This would increase the risk of a successful attack by structured query language (SQL) injection, which is a technique that exploits a security vulnerability in an application's software layer that allows an attacker to execute arbitrary SQL commands on the underlying database. SQL injection can result in data theft, data corruption, or unauthorized access to the system.
Buffer overflow, denial of service (DoS), and phishing are not directly related to the validation controls in a web application. Buffer overflow is a type of attack that exploits a memory management flaw in an application or system that allows an attacker to write data beyond the allocated buffer size and overwrite adjacent memory locations. DoS is a type of attack that prevents legitimate users from accessing a service or resource by overwhelming it with requests or traffic. Phishing is a type of attack that uses fraudulent emails or websites to trick users into revealing sensitive information or installing malware.
References:
Client-side form validation - Learn web development | MDN
JavaScript: client-side vs. server-side validation - Stack Overflow
SQL Injection - OWASP

A corrective control is a control that aims to restore normal operations after a disruption or incident has occurred. Executing emergency response plans is an example of a corrective control, as it helps to mitigate the impact of an incident and resume business functions. Separating equipment development testing and production is a preventive control, as it helps to avoid errors or unauthorized changes in production systems.
Verifying duplicate calculations in data processing is a detective control, as it helps to identify errors or anomalies in data processing. Reviewing user access rights for segregation is also a detective control, as it helps to detect any violations of segregation of duties principles. References: ISACA, CISA Review Manual,
27th Edition, 2018, page 64

Data classification is the process of organizing data into categories based on its sensitivity, value, and risk to the organization. Data classification helps to ensure that data is protected according to its importance and regulatory requirements. Data classification also enables data owners to make informed decisions about data access, retention, and disposal.
To implement a data classification program, it is most important to formalize data ownership. Data owners are the individuals or business units that have the authority and responsibility for the data they create or use. Data owners should be involved in defining the data classification levels, assigning the appropriate classification to their data, and ensuring that the data is handled according to the established policies and procedures. Data owners should also review and update the data classification periodically or when there are changes in the data or its usage.
The other options are not as important as formalizing data ownership when implementing a data classification program. Understanding the data classification levels is necessary, but it is not sufficient without identifying the data owners who will apply them. Developing a privacy policy is a good practice, but it is not specific to data classification. Planning for secure storage capacity is a technical consideration, but it does not address the business and legal aspects of data classification.
References:
ISACA, CISA Review Manual, 27th Edition, 2020, page 247
Data Classification: What It Is and Howto Implement It