If only (a^2+b^2+c^2) is written for ( (a+b+c)^2 ), which terms are missing?

Correct answer: (2ab+2bc+2ca)

The direct answer is B: the missing terms are 2ab+2bc+2ca. Begin with the square of a sum:
\((a+b+c)^2=(a+b+c)(a+b+c)\). Multiplying each term by every term gives a², b², c², ab twice, bc twice, and ca twice. Therefore,
\((a+b+c)^2=a^2+b^2+c^2+2ab+2bc+2ca\). Option A has the pair products but misses their required factor 2, so it is incomplete. Option B is correct because every pair appears twice in the multiplication. Option C, 3abc, is a three-variable product and belongs to neither the ordinary square expansion nor the required degree-two result. Option D is only the original sum, not the extra cross terms produced by squaring. A useful exam cue is: for three terms, write the three individual squares, then twice each of the three pair products. Check that every term has total degree two.