d
\(\begin{array}{l}
Given\,\vec u = 3\hat i + 4\hat j,\hat a = 0.4\hat i + 0.3\hat j,t = 10s\\
\vec v = \vec u + \vec at = 3\hat i + 4\hat j + \left( {0.4\hat i + 0.3\hat j} \right) \times 10\\
= 7\hat i + 7\hat j\\
\therefore \left| {\vec v} \right| = \sqrt {{7^2} + {7^2}} = 7\sqrt 2 \,units
\end{array}\)