Answer
b. between \$100 and \$200
Work Step by Step
John has been working as a tutor at \$300 a semester
When the pay to the tutor has been increased to \$400, the Producer surplus for John has increased by \$100
But, due to the increase in the pay, Emily enters into tutoring. Emily's costs of production might be anywhere above \$300 and below \$400. This means that the producer surplus for Emily is anywhere between \$0 to \$100.
Hence, the total producer surplus would be increased by \$100 to \$200
