VB.Net - Option Explicit

This setting does not exist in C#, because variables must always be declared
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The default is on and this is not included in your source code.
Determines if you have to declare all your variables before they are used.

C# requires all variables to be declared and there is no way of turning this off.
In C# all variables must be declared before they are used.

Explicit Declaration / Option Explicit

VBA is not a "strictly typed" programming language so you can get away with not explicitly declaring the data type of your variables.
This is very bad practice and leads to code that is slow and unreadable.
There is an option though that can force you to explicitly declare the data type of each variable and this should be switched on.
Check your (Tools > Options)(Editor tab, "Require Explict Declaration") and make sure it is checked.
Selecting this check box will automatically add the statement Option Explicit to any new modules (not existing ones).

Option Explicit 

When this statement is used you must explicitly declare all your variables before using them.
This ensures that a program will not run if it contains any variables that have not been explicitly declared.

You declare variables by using the "Dim" keyword.
Local variables are the most efficient because VBA frees up the memory when the procedure or function executes

Any variables that are not specifically declared are assigned as a "Variant" data type. VBA automatically converts the data to the proper type when it is used.

The variant data type generally wastes memory although there is another reason why all your variables should be explicitly declared.
This is related to preventing typing errors.
If you accidently misspell a variable name (which can happen) a new variable will not be created if your variables have all been explicitly declared.
This can lead to some nasty Run-time errors and unpredictable results.

Variables should always be declared explicitly as it ensures that spelling mistakes are picked up and ensures that memory is used efficiently.
The "Variant" data type uses a minimum of 16 bytes and is very slow. Using a list of comma-separated variables does not work.
You do not have to explicitly declare your variables, you can let Visual Basic do this for you.
Any variables that are created automatically have the "Variant" data type and can contain any type of data.
VBA automatically converts the data to the proper type when it is used.
You should explicitly initialise the variables and not rely on the system defaults.
You code will run a lot slower since a variant data type uses up more memory.