Like others, I answered A because it returned "the expected output": NULL. Option C returns an error:

Msg 208, Level 16, State 1, Line 1

Invalid object name 'dbo.Employees'.

I didn't put in the name of a valid table name in my database because that was not a part of the question. The question had the following as a "prequel" to all three of the queries:

declare @tablevariable varchar(100)

set @tablevariable = 'Employees'

So, for option C, I executed

declare @tablevariable varchar(100)

set @tablevariable = 'Employees'

DECLARE @sql nvarchar(4000), @params nvarchar(4000), @count int

SELECT @sql = N' SELECT @cnt = COUNT(*) FROM dbo.' + quotename(@tablevariable)

SELECT @params = N'@cnt int OUTPUT'

EXEC sp_executesql @sql, @params, @cnt = @count OUTPUT select @count

Therefore, option A returns the expected result (since the table variable is empty) and should be a correct answer.