The sentence 'Ich muss heute arbeiten.' means 'I must work today.' It correctly uses 'muss' to indicate necessity. Option 'b' is incorrect because 'muss' is not followed by a verb. Option 'c' is incorrect because 'muss' should not be used with 'gerne'. Option 'd' is incorrect because 'wir' should use 'müssen', not 'muss'.