Step 1: Read the sentence closely. The speaker doubts that Srikanth can handle the job "by himself," and gives the fact that he is "green" as the reason.
Step 2: When "green" is used to describe a person rather than a colour or a plant, it is an informal way of saying that the person is new to a task and has not yet built up the skill or experience needed to do it confidently on their own, as in phrases like "a green recruit."
Step 3: Test each option against this sense. "Childish" points to immature behaviour, which is not what the sentence is about. "Envious" means jealous, which has no connection here. "Indecisive" refers to an inability to make decisions, again unrelated to the doubt expressed. "Irresponsible" suggests carelessness or unreliability, which is close in tone but describes attitude rather than a lack of skill. "Inexperienced" is the only option that directly matches the idea of someone being too new to the work to be trusted with it alone.
Step 4: Since the sentence is explaining why Srikanth cannot manage the job by himself, the missing quality is a lack of experience.
Answer: (d) Inexperienced