I'm just (being) sad. / I've just been sad.

[sitifan]

Nothing serious. ________.
(A) I'm just sad. (B) I'm just being sad. (C) I've just been sad.
Which option or options are acceptable?
I think all are possible.

[5jj]

Sadness is not normally of short duration, so only A is likely.

[emsr2d2]

Nothing serious. ________.
(A) I'm just sad. (B) I'm just being sad. (C) I've just been sad.
Which option or options are acceptable?
I think all are possible.

A makes sense in the context of the rest of the statement and, in my opinion, is the only correct answer.

B is a possible statement but only with the alternative definition of "sad" - ie not cool, not trendy.
"Why are you looking at the Cliff Richard CDs?"
"I'm just being sad" OR "I'm just being a saddo!"

C is possible if "just" is taken to mean "only" instead of "recently".
"How have you been feeling recently?"
"I've been OK. I haven't been depressed. I've just been [a bit] sad."