Final answer:
The correct answer is 'd) Boon', which means something beneficial or lucky. 'Best', 'Bold', and 'Bane' do not fit the definition in this context.
Step-by-step explanation:
In the context of "Great 'B'-ginnings", the 4-letter word that refers to something beneficial or lucky is d) Boon. The word "boon" signifies a timely benefit or a stroke of good luck. Whereas, "best" is a superlative form of good, "bold" relates to courage or willingness to take risks, and "bane" stands for a source of harm or ruin - thus these options do not fit the definition sought. It's important to understand the meanings of words in order to identify the correct answer. The exercise of underlining the letters 'b' in provided words is a fun way to recognize and remember the spelling of the word "boon" among others.