To answer this question, you need to understand what are VB.Net identifiers and what are the rules for naming them. An identifier is a name given to a variable, constant, function, procedure, class, module, or any other user-defined item in VB.Net. The rules for naming identifiers are:

An identifier must begin with an alphabetic character or an underscore (_). An identifier cannot contain an embedded period (.). An identifier must not exceed 255 characters. An identifier cannot be a reserved keyword or a predefined object name. An identifier must be unique within its scope. An identifier can contain alphanumeric characters (A-Z, a-z, 0-9) and underscores (_). An identifier is case-insensitive, meaning that uppercase and lowercase letters are treated as the same. Let’s go through each option to understand why it is correct or incorrect:

Option A) are case sensitive. - This option is incorrect because VB.Net identifiers are case-insensitive, meaning that uppercase and lowercase letters are treated as the same. For example, myVar and MYVAR are the same identifier.

Option B) can begin with an underscore. - This option is correct because VB.Net identifiers can begin with an underscore (_), as long as they do not start with a number or a period. For example, _myVar is a valid identifier.

Option C) can begin with a number. - This option is incorrect because VB.Net identifiers cannot begin with a number. For example, 1myVar is an invalid identifier.

Option D) Both a and b. - This option is incorrect because it combines two options that are not both correct. Option A is incorrect and option B is correct.

Option E) All of the above. - This option is incorrect because it includes all the options that are not all correct. Options A, C, and D are incorrect and option B is correct.

The correct answer is B. This option is correct because VB.Net identifiers can begin with an underscore (_), as long as they do not start with a number or a period.