Step 1: Dimension analysis.
The force is given by:
\[
F = at + bt^2
\]
The dimensions of force \( F \) are [MLT\(^-2\)].
Step 2: Dimension of \( a \).
For the term \( at \) to have dimensions of force, the dimension of \( a \) must be [MLT\(^-3\)].
Step 3: Dimension of \( b \).
For the term \( bt^2 \) to have dimensions of force, the dimension of \( b \) must be [MLT\(^-4\)]. Thus, the correct answer is (2).