In the given passage few words are missing, which are followed by serial numbers. Choose the appropriate word from the given options that can fit into the given blank without altering the intended meaning.

Economic development and growth are supposed to ______________ (16) the employment situation in a country. Much of development literature _______________(17) this idea as a major expectation at least from the long-run perspective. It is also the case that the greater the amount of ________________ (18) labour in an economy and the greater the ________________(19) of underemployment, the slower will be the pace of change in the employment structure towards more regular and protected waged forms of employment. In fact, one can only give a proper perspective to the misunderstood policy challenges of __________________ (20) in a developing economy by first examining these matters at the broadest level with existing data.

The correct option to fill in blank (19) is extent.

Step 1: Understand the context of the sentence
The sentence reads: "It is also the case that the greater the amount of ... labour in an economy and the greater the ________________(19) of underemployment, the slower will be the pace of change in the employment structure..."
The blank requires a noun that refers to the degree, magnitude, scale, or scope of underemployment present within an economy.

Step 2: Analyze the given options

1. hustle: Refers to busy movement, activity, or energetic action. It does not fit contextually with "underemployment".
2. stretch: Means to extend or spread out, or a continuous period/area. It does not collocate appropriately with "underemployment".
3. reference: Refers to a mention, citation, or source of information. It makes no sense in describing the magnitude of an economic problem.
4. favour: Means approval, support, or a helpful act. It is logically incorrect to speak of the "favour of underemployment".
5. extent: Means the area, degree, or severity to which something exists or affects a situation. The phrase "the extent of underemployment" is grammatically correct and precisely conveys the degree/level of underemployment in an economy.

Conclusion:
Therefore, the word extent is the most suitable choice to fill in blank (19).