Explanation
Correct code block:
transactionsDf.select(col("storeId").cast(StringType()))
Solving this question involves understanding that, when using types from the pyspark.sql.types such as StringType, these types need to be instantiated when using them in Spark, or, in simple words, they need to be followed by parentheses like so: StringType(). You could also use .cast("string") instead, but that option is not given here.
More info: pyspark.sql.Column.cast - PySpark 3.1.2 documentation
Static notebook | Dynamic notebook: See test 2