D. Polar bonds and asymmetrical structure.
To be polar, a molecule must have polar bonds and their bond dipoles must not cancel. That is, the molecule must have an asymmetrical structure.
For example, water is a bent molecule. The O-H bonds are polar, with their bond dipoles pointing toward the O atom (See Figure 1).
The x-components of the dipoles cancel, but the y-components reinforce each other.
A. and B. Wrong. If the bonds are nonpolar, the molecule is nonpolar.
D. Wrong. If the polar bonds are arranged symmetrically, the molecule is nonpolar.
For example, carbon dioxide is a linear molecule. The C=O bonds are polar, with their bond dipoles pointing towards the O atoms (see Figure 2).
The bond dipoles are equal, but in opposite directions, so they cancel each other.
Carbon dioxide has polar bonds, but it is a nonpolar molecule because of symmetry .
technetium is the most reactive. this is followed by silver. the least reactive out of these three elements is strontium.