Step 1: Grasp development progresses from using both hands, to one hand, to a refined finger-thumb pincer.
Step 2: The earliest of these, the two-handed (bidextrous) reach, is achieved around 4 months of age. By 6 months the baby reaches with a single hand (unidextrous), and pincer grasp matures by the end of the first year.
Step 3: Mapping the milestone to the options, the bidextrous grip corresponds to 4 months, ruling out 5, 6 and 7 months.
\[\boxed{\text{4 months}}\]