Acquired immunity is characterised by:

The correct answer is Option (1) → Pathogen specificity and memory

Immunity is of two types. Innate immunity is present from birth and acts against any foreign agent, so it is non-specific. Acquired immunity develops after the body meets a pathogen. It is aimed at that one pathogen and it remembers it. The first meeting gives a weak primary response; a later meeting with the same pathogen gives a much stronger secondary response.

Option (1) Pathogen specificity and memory — Correct. Acquired immunity is pathogen specific and is characterised by memory. These are its two defining features.

Option (2) Non-specific response and memory — Incorrect. A non-specific response is the mark of innate immunity. Innate immunity does not have memory.

Option (3) Pathogen specificity and low intensity response — Incorrect. Specificity is right, but low intensity describes only the first (primary) response. The secondary response is highly intensified, so low intensity is not a feature of acquired immunity as a whole.

Option (4) Non-specific response and high intensity response — Incorrect. Acquired immunity is not non-specific. A high-intensity response is seen only on a second meeting with the same pathogen.

NCERT Extract:
Acquired Immunity – “Acquired immunity, on the other hand is pathogen specific. It is characterised by memory. This means when our body encounters a pathogen for the first time it produces a response called primary response which is of low intensity. Subsequent encounter with the same pathogen elicits a highly intensified secondary or anamnestic response.”
Innate Immunity – “Innate immunity is non-specific type of defence, that is present at the time of birth.”