Marine biologists have long been fascinated by the intelligence of octopuses. After observing complex problem-solving behaviors in laboratory settings...

Let's begin by understanding the meaning of this sentence. We'll use our understanding of pause points and segment the sentence as shown - understanding and assimilating the meaning of each segment bit by bit!

Sentence Structure

• Marine biologists have long been fascinated by the intelligence of octopuses.
• After observing complex problem-solving behaviors in laboratory settings,
  • many researchers chose (?) these animals in their natural ocean habitats
    • to better understand their cognitive abilities.

What varies: study / to study / studying / studied

Understanding the Meaning

The first sentence sets up the context:

• 'Marine biologists have long been fascinated by the intelligence of octopuses.'
  • Scientists have been really interested in how smart octopuses are.

Now the second sentence gives us more detail:

• 'After observing complex problem-solving behaviors in laboratory settings,'
  • After watching octopuses solve difficult problems in labs,

Now here's where we need to fill in the blank:

• 'many researchers chose _____ these animals in their natural ocean habitats'

Let's look at our choices:

• We have different forms of the verb "study"
  • study (base form)
  • to study (infinitive)
  • studying (gerund)
  • studied (past tense)

What do we notice about the verb "chose"?

• The verb "choose" has a specific pattern it follows
  • When you use "choose," you follow it with "to" + verb
  • We say "I chose to go" or "they chose to investigate"
  • NOT "I chose go" or "they chose going"

So we need: to study

The sentence continues:

• 'to better understand their cognitive abilities'
  • This tells us WHY they chose to study the octopuses in the ocean - to better understand how their minds work.

The complete meaning:

• After watching octopuses solve problems in labs, researchers decided to study them in the ocean to learn more about their intelligence.

The correct answer is B: to study

GRAMMAR CONCEPT APPLIED

Verbs That Require Specific Forms to Follow Them

Certain verbs have conventional patterns - they require specific forms to come after them. The verb "choose" is one that requires an infinitive (the word "to" plus the base form of a verb):

Pattern: choose + to + [verb]

Examples:

• Scientists chose to investigate the phenomenon further
  • NOT: chose investigate
  • NOT: chose investigating
• She chose to pursue a career in medicine
  • NOT: chose pursue
  • NOT: chose pursuing
• They chose to study marine biology
  • NOT: chose study
  • NOT: chose studying

In our question:

• "many researchers chose _____ these animals"
• Following the pattern: chose + to + study
• Answer: chose to study

This is an example of what's called an idiomatic expression in grammar terms - a conventional way that certain words are used together in English.