The correct answer is (B) foremost part, spearhead; the leading portion of an advancing army; advance guard. Also, the forefront of any movement, field, or activity; the leaders of any movement, whether intellectual or political.

Vanguard comes from Middle English vandgard, Old French avangarde, variation of avant-garde.

Examples of use:

• The vanguard of the advancing army consisted of cavalry units and infantry.
• He prided himself on being in the vanguard of new technology in his field.