Past Perfect Continuous states that something had been happening before a related event in the past. For example:
“Ian Thorpe had been swimming for 15 years before winning the Gold medal”
The way that we create a Past Perfect Continuous sentence is
Subject (the person we are talking about) – had been – present participle (verb with -ing at the end)
“I had been washing for 10 minutes before the rain started.”
In your sentence, you can see that we are talking about the past “I was very happy when Emily came back from Poland”.
The gap fill gives us multiple options about the verb “missing” which means to feel that we wish someone (who isn’t with us) is with us. So we would miss Emily before she came back. This is why C. – I’d been missing is correct. Because it’s Past Perfect Continuous. Something that had been happening before another event in the past.
