LINQ (Language Integrated Query) is a feature of .NET that allows you to write queries to manipulate data from various sources, such as arrays, collections, databases, or XML files. LINQ provides a set of methods that can be used to perform different operations on the data, such as filtering, sorting, grouping, aggregating, or transforming.

The LINQ method that is used to filter data in a List or DataTable based on a condition is Where. The Where method returns a new collection that contains only the elements that satisfy the specified condition. The condition is usually a lambda expression or a delegate that takes an element as a parameter and returns a Boolean value. For example, if you have a List of integers called numbers, you can write:

var evenNumbers = numbers.Where (x = > x % 2 == 0);

This expression returns a new List that contains only the even numbers from the original List. Similarly, if you have a DataTable called employees, you can write:

var highSalary = employees.AsEnumerable ().Where (row = > row.Field < decimal > (\ " Salary\ " ) > 100000);

This expression returns a new DataTable that contains only the rows where the Salary column is greater than 100000. The AsEnumerable () method is used to convert the DataTable into an IEnumerable < DataRow > , which is required for using LINQ methods.

The other options are not LINQ methods that are used to filter data in a List or DataTable based on a condition. Option A, Select, is a LINQ method that is used to project or transform data from one form to another. It returns a new collection that contains the result of applying a function to each element of the source collection. Option B, GroupBy, is a LINQ method that is used to group data by a common key or attribute. It returns a new collection that contains groups of elements that share the same key value. Option C, OrderBy, is a LINQ method that is used to sort data in ascending order by a specified criterion. It returns a new collection that contains the elements of the source collection in sorted order.