Step 1: State the concept.
Gelatin is made from collagen, and the way the collagen is pre-treated decides the gelatin type. Acid treatment gives Type A, while alkali (lime) treatment gives Type B. The isoelectric point ($pI$) is the pH at which the protein carries no net charge.
Step 2: Reason about the chemistry of alkali processing.
Alkali treatment hydrolyses the amide groups of asparagine and glutamine into aspartic acid and glutamic acid. This adds extra carboxylic acid groups, which makes the protein more acidic and pushes its $pI$ down into the acidic region.
Step 3: Recall the known ranges.
Type B gelatin has its isoelectric zone around $pH$ $4.7$ to $5.2$, whereas Type A (acid processed) sits higher, near $pH$ $7.0$ to $9.0$.
Step 4: Eliminate the wrong options.
Option 1 ($pH$ $9.0$) and option 3 ($pH$ $7.0$) belong to Type A, not Type B, so both are rejected.
Step 5: Eliminate the remaining outlier.
Option 4 ($pH$ $1.2$) is far too acidic for any gelatin isoelectric zone, so it is discarded.
Step 6: Conclude.
The only value matching the alkali-treated Type B gelatin is $pH$ $4.7$, which is option 2.
\[ \boxed{\text{Option 2: } pH\ 4.7} \]