Why/When are you so sad?

Solution:

To answer the question "Why/When are you so sad?", we need a reason or a time. Let's look at the options.

1. Option 'a' "Valentine's Day!" is a specific day and could be a reason for sadness if something negative happened on that day, but it's not a direct answer to 'why' or 'when' someone is sad.
2. Option 'b' "It's on 16th June." states a date, which answers 'when' something happens, but not 'why' someone is sad.
3. Option 'c' "By bus." answers 'how' someone travels.
4. Option 'd' "Because I lost my new watch." provides a direct reason for sadness, answering the 'why' part of the question.
5. Option 'e' "They're fairy penguins." is an unrelated statement.
6. Option 'f' "We can!" is an affirmative response to a capability question.
7. Option 'g' "It's in the cupboard." answers 'where' something is.

Therefore, the most fitting answer to "Why are you so sad?" is "Because I lost my new watch." which corresponds to option 'd'.

Answer: d Because I lost my new watch.