The correct option is
B Light
When two words are brought together to yield a new meaning, they are called compounds, or compound words.
Option B: 'Moon' forms a new word, followed by the word 'light': moonlight.
'Moonlight' means 'the light of the moon'.
Hence, option B is correct.
Options A, C and D: 'Moon' does not form a compound word with 'bed', 'hood' or 'port'.
The words 'moonbed', 'moonhood' and 'moonport' do not exist in standard English language.
Hence, options A, C and D are wrong.