Consider the species CH$$_4$$, NH$$_4^+$$ and BH$$_4^-$$. Choose the correct option with respect to the three species:

We need to determine if CH₄, NH₄⁺, and BH₄⁻ are isoelectronic and whether they all have tetrahedral structures. CH₄ has C with 6 electrons and 4 H atoms contributing 4 electrons, for a total of 10 electrons. NH₄⁺ has N with 7 electrons and 4 H atoms contributing 4 electrons minus 1 for the positive charge, also totaling 10 electrons. BH₄⁻ has B with 5 electrons and 4 H atoms contributing 4 electrons plus 1 for the negative charge, again totaling 10 electrons. Therefore, all three species are isoelectronic with 10 electrons.

Each species features a central atom bonded to 4 hydrogen atoms with no lone pairs on the central atom, which corresponds to sp³ hybridisation and a tetrahedral geometry. Consequently, CH₄, NH₄⁺, and BH₄⁻ all adopt tetrahedral structures.

The answer is Option B: They are isoelectronic and all have tetrahedral structures.