In ( (a+b+c)^2 ), from where is the (ab)-term formed?

Correct answer: From (2ab)

When a sum of several terms is squared, every term is multiplied by every term. For three terms, the identity is \((a+b+c)^2=a^2+b^2+c^2+2ab+2bc+2ca\). The product of \(a\) and \(b\) appears twice: once as \(a\times b\) and once as \(b\times a\). Since both are like terms, they combine to give \(2ab\). Thus option B identifies the source of the ab-term.

The terms \(a^2+b^2\) are square terms, not the cross-product term, while \(abc\) does not occur in a square of a sum. Likewise, \(b^2+c^2\) contains only square terms for those two variables. Expanding the brackets or recalling the identity leads directly to the same result. Therefore the ab-term is formed from twice the product of a and b, namely \(2ab\), so option B is correct.