Because/Because of

"B" is correct.

'Because' means 'for the reason that'. If you used 'because' in that sentence, the clause would be left sort of dangling, incomplete in meaning.
'Because of' means 'by reason of ' or 'as a result of'.

'Because' can sometimes be replaced by 'as' and 'since'. If you try those, the sentence will make no sense.

'because of', on the other hand, is similar in meaning to 'owing to'.

You could rephrase the sentence as "Trees need more careful planning than other crops do, owing to the time and area involved in the cultivation of a forest."

Miriam