Concept:Express all numbers as powers of the same base (7) to compare exponents.Explanation:343=73 and 49=72. Also 7=71/2. So the equation becomes:73×(71/2)n=(72)4⇒73×7n/2=78 Using am×an=am+n, we get 73+n/2=78. Since bases are equal, equate exponents: 3+n/2=8. Solve: n/2=5⇒n=10. Answer:n=10 (Option B).